You can buy tickets online, from a travel agent in Greece, or you can wait and get your tickets when you arrive at the port. There are usually enough tickets to the islands even up to the time of departure. The only time you absolutely must book ahead is if you are traveling with a car, or if you are traveling in the busiest time of the year (mid July to the end of August). In any case, you need to have your tickets before you embark the ferry to a Greek port.
Buying while talking with a travel agent, either on the phone or in person, is the easiest way. You can communicate all your needs, and the agent can check all the options. The main port of Greece, Pireas, and all the ports around the country are lined with a multitude of small ticket offices that would be happy to sell you a ticket on the spot. They are quick and efficient and the are reliable. Not all ticket agents can sell tickets for all ferry companies, so check with several of them if you want to explore all your options. Usually the signs outside their offices indicate which companies they sell tickets for and for which destinations. Most of these agents will accept major credit cards, but we've run into several who only accept cash (usually in island ports).
Another way to buy your tickets is to search online for ferry tickets to the Greek islands. At the time of this writing (August 2011) the online purchasing system lags behind in convenience. You can search some of the available online ticket agents (a google search will give you a page full of them) and their database will ask you to enter your information before it provides you with available ferry availability. From my experience, the system will store your information and then a travel agent will verify and send you an email with a "proposal" that includes the price. You then have a day or so to accept or regect the offer and to ask questions. The system is not what you would expect from an online system (being slow and requiring emailing back and forth), but it is adequate. If you do buy your tickets online, you have to pay with a credit card, and you can receive your tickets at home via currier (for a fee, and only if there is enough time before departure), by visiting the ticket agents office, or you can pick them up at the ferry company's office at the port when you arrive before embarcation.
When you buy your tickets, you need to make several decisions. First you need to choose a class. The cheapest ticket you can buy is "deck". This means that you travel without accommodation, simply by sitting around on the deck, or by wandering around from lounge to lounge. In reality, most passengers who buy deck tickets claim a spot on board and "camp" there for the duration of the trip. Some bring a full tent, some wrap themselves in a sleeping bag, some open up a blanket where the whole family sits, and some simply lay down on the floor. While "deck" tickets only allow passengers to "camp" on the open deck, in reality most occupy a table or a chair in a lounge and stay there for the whole trip. In addition, the ship's hallways, stairwells, and any other open space is full of bodies that try to keep themselves comfortable until they arrive to their destination.
If you want a more comfortable place to lay down and sleep, you can buy tickets for an airline style seat. These are very similar to first class airplane seats lined up like an airplane in large rooms on the ferry. These seats are large and roomy, with ample space to fully extend the foot rest and they can be very comfortable for sleeping.
Both deck ticket holders and airline-style seat passengers share the same public bathrooms.
If you want to sail in more comfort to the Greek islands, for a higher fee you can purchase a cabin. Greek ferry cabins are very similar between companies and offer clean sheets and towels, private bathrooms and ample space to stretch out. They can fit up to four passengers in bunk beds, and you can opt for an interior (no windows) or an exterior cabin with a window for an extra fee. When hou purchase an exterior cabin, make sure it's not one overlooking an open deck because then you will be looking at the sea throuh several of the campers outside, and they will be looking at the interior of your cabin (in such case you might be keeping the window shades closed which would defeat the purpose of paying extra for an exterior cabin.
The ferries in Greece exist to accommodate passengers and to transport goods to and from the islands. As such, the bulk of their hull is dedicated to fitting lots and lots of cars and trucks. Out of ten decks, five or six would be garrages. You will pay a ticket for your vehicle and for each passenger, including the driver.
If you have a pet, you can purchase a "pet cabin" in most ferries. These are exactly the same as every other cabin on the ferry but they don't have a carpeted floor. They are just as clean as the other cabins, and there is usually a small fee to book them. If you purchased a ticket not knowing that there was a pet cabin option, you can ask to be switched to a pet cabin once you embark. Ask the at the reception if there are pet cabins available to upgrade. If you don't have a cabin, you can keep your pet on a leash on the open decks, or you can opt to put it in one of the "kennels" available. The kennels are metal boxes that are reasonably clean.
Another consideration when buying tickets to the Greek islands is when to travel. For short trips up to 4 hours (islands in the Cyclades) only day trips are available. But for islands that are farther away from the mainland, ferries offer overnight trips. If you opt for an overnight trip, purchasing a cabin to get some sleep is probably a better option. On overnight ferry trips you should be mindful of the arrival time to your island because sometimes the ferry might dock at your island at an odd hour when all hotels are closed and transportation is not available. In such case, it might be wise to have booked a hotel and to make arrangements with the hotel to check in as soon as you arrive, or to make other accommodations. Often hotels would let you check in at odd hours in the morning, but they will charge you for a whole day so make arrangements ahead of time.
As you plan your trip and buying your ferry tickets keep in mind that in heavy weather the ferries are not allowed to sail. If you have to catch a flight out of Athens, plan your return ferry trip a couple of days earlier so you don't have to worry about it if your ferry ride gets delayed. Such heavy weather that cand delay the largest ferries in the summer in Greece is rare but it does happen.
The last thing you should consider when buying ferry tickets to the Greek islands is the kind of ferry you want to use. Read the descriptions below to get an idea of what's available, but in general, the faster the ferry, the more expensive the ticket will be.
Now that you have your tickets to your favorite island you should give some thought to your ferry ride. If you have a car, you can leave all your belongings in your vehicle and only pack a small bag with the necessary items for the trip, keeping in mind that you won't have access to your car once the ferry sails. While crime aboard the ferries is extremely rare, you should not leave any valuables in your car and you should take the same precautions in you cabin like you would take for a hotel room. Pack a long sleeve garment in your day pack because the interior of the ships is air conditioned and it feels chilly. On night trips, the temperature on the deck drops dramatically as well.
If you don't have a vehicle, you can leave your suitcase or large bag in a cage at the entrance of most ferries so you don't have to haul it over the ship's stairs and crowded hallways. You can pick it up again during dissembarcation.
Ferries in Greece are extremely punctual with their departure times, so plan to get to the ferry at least an hour before embarcation. Most ferries sail at specific times that coinside and the traffic jams outside the port can delay your arrival considerably. If you bought "deck" tickets, you definetly want to get to the ferry early so you can stake a comfortable spot for yourself before the crowds arrive.
If you are traveling with a car, get close to the ferry and then let all the passengers out to get to the ferry by foot. Arrange for a meeting place on board (your cabin, the pool, but not the reception where everyone else crowds at that time), drive your car following instructions from officials and crew members and you'll be parked in no time. If you are traveling on foot, find the side entrance to the ferry (usually right next to the gate where the cars enter), mind the traffic and get on board. Each passenger should have their own tickent in hand as you step on board because an officer will take your stub and direct you onward.
If you paid for a cabin, head for the reception where a stewart will escort you to your cabin (a gratuity of 1 euro should be sufficient). From there you can relax and enjoy the trip. Elecricity in the cabins is standard 220V, but in the bathroom you will find 110V outlets.
If you have deck tickets, you should decide where you will spend the entire trip (inside or out). Most Greek passengers pick a table in one of the bars and stay there with their whole family, and this is probably the best way to go. No one will press you to buy drinks all the time, and no one will force you to leave the table. You cannot lay down or open a sleeping bag, but sitting on chairs is allowed. If all tables are taken by the time you get there, look around the tables by the pool. The areas around the pool are usually sheltered from wind and sun and can be comfortable, even though the temperature will range from very hot during the day, to cold during the night. If all fails, find a quiet corner in a hallway or an open deck and make the best of the trip. If you travel with a pet that you want to keep with you at all times, find a nice spot on the deck and keep your pet on a leash or in it's carrying cace. If you "camp" in the interior, you have to put your dog in the kennel.
Food on the ferry can be expensive so don't order without checking the prices first. A dish with meat can cost you 12 euro at the self-service restaurant, but a spaghetti dish can be half that price. If you are on a budget, opt for the sandwiches and pies sold at the bars, or better yet, bring your own food abroad. No one will complain if you eat your own meal at the bar or on the decks.
In terms of entertainment, the options are few. All ferries offer wireless internet for a fee (usually 3 euro per hour) and connectivity is good at the designated hot spots, but intermitent elswhere, including cabins. If you rely on Mobile Internet for your connectivity keep in mind that there is no 3G in open sea and GPRS internet speed will be painfuly slow. Many ferries have an "internet corner" with available computers on desks that you can use for a fee.
Cellphone connectivity is usually good between islands with very few dead spots in open sea. If your route takes you close to neighbouring countries (Turkey, Albania, Italy) make sure you are not connected through a foreign network that kicks in high roaming fees for your account.
There are electrical outlets in the hallways and lounges throughout every ferry (220V) that you can use to charge your appliances (computers, cellphones and the such). If you have deck tickets and need access to electricity, set your camp near an electrical outlet.
All ferries to the Greek islands, especially the ones that perform the longer trips to Crete an the Dodecanese have a swimming pool on the open deck. They fill it with salt water shortly after the ship leaves the port and empty it about half hour before arrival to the destination. The pool is small, so don't plan on doing laps, and for children there is usually a kid's pool in the vicinity. There are showers close to the pool but no towels are available so if you plan to swim, pack your own.
All ferries to the Greek islands are similar in other ameneties. They all have a couple of bars inside and at least one by the pool, an arcade where young adults can play electronic games, a kid's play area for younger children, and a couple of shops with gifts and apparel. You can get snacks and magazines in these shops, but not newspapers. These shops have limited hours of operation an they are not open for the entire trip. Ferries that travel between Italy and Greece (see getting to Greece from Italy here) even have a casino aboard.
Disembarkation is usually a straight forward affair. Listen for the announcements and follow the crowds to the exits.
In trips where the ferry makes intermediate stops, these are very short. If you are disembarking anywhere before the ferry's final destination be prompt to the exit or you might miss your island. You will be amazed how efficient and quick embarkation and disembarcation is in such stops.
If you travel with a car, get everyone in the car and exit the boat all together if you can because it's hard to pick up passengers once you exit the ship. Usually the officials will urge you to move on and won't let you stop near the boat to pick up your passengers.
Several kinds of ferries can take you to the Greek islands, and the kind you choose depends on the distance, availability and budget of your trip.
The newest members of the ferry breed are the "fast ferries" that sail to the islands in half the time of the conventional ferries, but they also cost twice as much.
The sight of the spilled water as these ferries speed on the surface is awe inspiring, and even thought there are signs that no one is allowed on open deck during sailing, many prefer to stay on the cramped open air back deck to the enclosed interior - although the wind of the speeding ferry and the sea spray it generates make staying there for prolonged periods of time uncomfortable.
The fast ferries and catamarans come in several sizes. Tiny "flying dolphins" serve the short distances between neighboring islands (like the Saronic gulf islands), while larger ones serve the whole Aegean theater.
A ticket to the fast ferries buys you an airplane-type seat with ample room around to cross your legs and lay back in comfort. You can also choose seats with a table for a little extra, while "preferred" class is also available.
While these fast ferries take travelers to their destination fast, they offer none of the mystique of sailing. You can't feel the clean sea breeze, you can't feel the cool sea, and you can't sleep under the stars to the tune of some stranger's guitar.
They feel more like a very comfortable airplane on water.
They are the newest members of the Greek passenger fleet, and they are very safe, reliable, punctual, and clean. They also get in and out of ports very fast, and like the conventional ferries, they stop at several islands on a single trip. They are also the most expensive option of traveling to the Greek islands.
These ferries serve the majority of the Greek islands and sail between Italy and western Greece. They are reliable, and very punctual. Depending on the length of the trip and the owner company, the service aboard varies slightly from ferry to ferry.
Onboard they offer cabins, restaurants, bars, arcades, and even personal DVD player and movie rentals.
They are usually very comfortable and can handle rough seas with grace. Parking in their bellows is usually a painless affair. Usually, the port police would direct all passengers to exit the vehicles before they enter the garage, although many times the passengers can ride into the garage with the vehicle.
With few exceptions, the ferries that serve the Aegean islands tend to stop at multiple destinations, making "island hopping' and easy affair. These stops are very short and efficient. It often takes only fifteen minutes to enter a port, disembark, and pick up more passengers and cars before the ferry sails to the next island.
For long overnight trips most passengers prefer to reserve a cabin to sail in comfort. Those who travel on a budget opt for "deck" tickets which buys them a spot on the deck.
"Deck" ticket holders usually settle on a little spot on the deck or the lounge of the ship, and late at night the atmosphere can be festive when tourists unroll their sleeping bags and unpack their guitars for a romantic night under the stars. Most Greek island hoppers make this one of their central memories of their trips. It's usually all fun unless you ride on a ferry with a group of two hundred high school campers, when riding on the deck can be a taxing affair.
For long trips on these ferries it is best to board early and to mark a comfortable spot that would accommodate sitting and even sleeping for the long trip. Later in the day all the best spots will be taken, and there is nothing romantic about spending the whole night sitting on an uncomfortable chair.
For short distances, like between Pounta and Elafonisos, usually the flat ferries are available. They are generally very old and have been the workhorses of the passenger fleet for almost half century. They are usually reliable, but tend to rock a great deal in even moderate weather. Their schedules are subject to cancellations when the sea gets slightly rough.
The service onboard these ferries ranges from bare-bones to crude. Usually there is a small bar serving snacks and refreshments are inflated prices, and most of the passengers prefer to stay on the open deck benches for the short trips.
Boarding one of these ferries with a car can be a nightmare for novice and non-Greek drivers. The crew usually tries to pack the cars as tight as possible and a number of them direct drivers to back-up into the ship, and then they request that the driver abandons all control to their signals. It all works pretty well at the end assuming you can exit your vehicle before the next one is parked few inches from your door forcing you to stay in you car - which is not allowed - or to climb out your window.
Most Greek drivers are well acquainted with these ferries - mainly from their previous service in Rio/Antirio - and many have some interesting stories to tell about them. They operate between most large Greek islands that are within a short distance from mainland. These ferries are often (but not always) the cheaper option, and you will find them connecting short distances between Pireas-Aegina, Perama-Salamina, Igoumenitsa-Corfu, Lefkada-Kefalonia, among other routes.