This page explains what each class actually gives you. Names and availability vary by operator and by vessel, so treat these as the shape of the system rather than a fixed menu, and read what your specific booking includes.
Deck
The cheapest, and it means what it says: you have passage, and you sit where you can find a space or stand on the open deck.
On a short daytime crossing in good weather this is not a hardship, it is the best seat on the boat. You are outside, the islands go past, and nobody is between you and the view.
On a long crossing, in bad weather, at night or with luggage and children, it is a poor choice. There is no reserved space and no guarantee of one when the boat fills.
Economy salon
A step up: access to an indoor lounge with seating that is not numbered. You sit where you find a place, indoors, with a snack bar and somewhere to put a bag.
This is the sensible default for medium crossings. It costs little more than deck and it removes the two things that make deck unpleasant, which are weather and nowhere to sit when the boat is busy.
Air seat
A reserved, numbered seat in an air-conditioned indoor lounge, of the kind you would recognise from an aircraft. This is the class most people should buy for anything over about three hours.
The reservation is the point. On a busy summer sailing the difference between a numbered seat and a hopeful one is the whole journey. Operators typically tier these into standard, business and higher grades, with the upper tiers on quieter decks and with better seats.
Business seats cost meaningfully more than economy and buy space, a quieter lounge and often a better view. Whether that is worth it is a personal judgement; the numbered seat itself is the part that is nearly always worth it.
Cabins
On the long routes and the overnight sailings, cabins are available and they change the trip completely.
Inside cabins have no window. Outside cabins have a porthole and cost more. Both come in two, three and four-berth configurations, and some vessels have larger or higher-grade rooms.
The rule of thumb: on an overnight crossing, a cabin is not a luxury, it is the difference between arriving able to drive and arriving unable to. On the long north Aegean runs such as Athens to Chios, book one.
Which to buy
Under two hours. Deck or economy. Do not overthink it.
Two to four hours. Economy salon, or an air seat if the sailing is busy.
Four hours or more, daytime. Air seat, reserved. This is the one where people economise and regret it.
Overnight. Cabin, and book it early because they go first.
With a car. The vehicle space is booked separately and is the thing that actually sells out in August. What to book ahead covers the order in which Greek travel fills up, and taking a car on a Greek ferry covers the vehicle side.
Fast boats are different
Worth a note, because the class structure barely applies. The high-speed catamarans that run many Cycladic routes are seated aircraft-style throughout, often with no open deck access at all while under way.
That means no sitting outside, and it means a rougher ride in wind than a conventional ferry, because a fast boat is lighter. If you are prone to seasickness, the slower boat is the kinder one despite taking longer. Greek island ferries covers the vessel classes in full.
Where this leaves you
Deck for a short hop in good weather, economy salon for the middle distances, a reserved air seat for anything long enough to matter, and a cabin for anything overnight. The mistake to avoid is buying deck for a nine-hour crossing because it was the cheapest box on the page. Check what your specific operator calls each class, because the names move even where the structure does not.
Quick answers
What does a deck ticket mean on a Greek ferry?
Passage without a reserved seat. You sit where you can find space or stand on the open deck. It is genuinely pleasant on a short daytime crossing in good weather and unpleasant on a long one, at night, or when the boat is full.
Should you book an air seat?
For any crossing over about three or four hours, yes. An air seat is a reserved numbered place in an air-conditioned lounge, and on a busy summer sailing the reservation is the difference between a comfortable journey and standing.
Do you need a cabin on a Greek ferry?
On an overnight sailing, effectively yes: it is what allows you to arrive rested rather than wrecked. Inside cabins have no window and cost less than outside ones with a porthole. Book them early, because they sell out first.
Are the fast boats more comfortable?
Not necessarily. High-speed catamarans are seated aircraft-style with little or no open deck access while under way, and being lighter they ride more roughly in wind. If you are prone to seasickness, the slower conventional ferry is usually the kinder choice.