cspk, Lee's advice is the best ... and some of the worries posted would not have been necessary if you had stated up front WHEN you would be traveling. Comments :
• IF you book online, this may involve tickets sent to you overseas (costly & a hassle), OR delivered to an Athens hotel upon your arrival (but what if there's a Glitch? Panic!), OR having to go to ticket kiosk at pier to pick them up before departure. In my 12 trips (all in May-June or September), I've preferred to buy at any handy agency near hotel in Athens or an island, 1 - 3 days before. They may charge €1 convenience fee, big deal.
- Capacity depends on ferry type: #1 -- conventional ferries such as BLUE STAR line (1200-1800 passengers, 250+ trucks & vehicles) #2 -- Car-carrying catamarans: large-ish, 500+ passengers #3 - Passenger-only catamarans -- 350-450. (All catamarans are reserved-seating-only. Naturally, if #3 is the only ferry service & travel is at a busy time, plan ahead. Personally booking a few days ahead, I've never experienced a sold-out in May or Sept.
• Booking a week or more in advance on Blue Star type ferries is almost never necessary except in HIGH season (July-Aug), and of course, any time around EASTER or Aug 15 -- 2nd biggest holiday in Greek year. Also, check Greek calendar online for the few 3-day holiday weekends such as Pentecost (Holy Spirit) weekend, usually late May-early June. Other times to be alert for heavy bookings: Outbound, Athens to Islands on Fridays, May thru September, and Inbound from Islands to Athens, summer Sundays.
• The sold-out situation reported by Rosalyn was for a special reason. The 2 ferries mentioned are the ONLY service between Crete and Santorini... and while they're fairly big catamarans, they just seat 400-500, not 1500.
• For big-ferry short trips such as Naxos - Santorini, no point in anything but economy ... you'll want to be on deck. If doing that long trip to/from Piraeus (5 hrs to/from Naxos), consider a reserved-economy (also called "aircraft seating"). You can sit on deck as long as you like, but if/when you want to be inside to read/nap, you have a guaranteed numbered seat.
TIP #1 - in May, early June, or September, large ferries can be at least 1/3 empty. Don't tell anybody, but in those months, buy a plain economy ticket, then just wander inside, look for empty seats and usurp them. If for some reason it looks full-ish, you can just flag a staff person and upgrade an Economy ticket by €5, for a numbered seat.
TIP #1 - On blue Star, best reserved seats are on either side, in smaller compartments, by the windows, quieter. The central seating is more like a movie theatre, up to 400 seats, divided by aisles into thirds, TV on nonstop, brightly lit. If you book online you cannot specify side seating, but you CAN if you book in person at an agency. If you're a worrier, directly on arrival, you can ticket-shop right at the Airport at Either Amphitrion or Pacific travel agencies, open 24/7.
TIP#2: Deck seats on Blue STar ferries are first-come-first-served, so strategize. Be near the front of the line waiting at dock. When ferry officers signal boarding, step lively! You're directed to stash big bags on steel racks on the vehicle level (perfectly safe; just take with you any daybags with $$, camera, valuables). If there are 2 of you, assign one to bag-stowage, the other scampers up the steps to snag 2 best deck seats.
TIP #3: Suppose you have a reserved-economy seat on one of those (rarely) fully-booked ferries returning to Piraeus on a big weekend. After watching sunset on deck, you come inside to your numbered seat and... SHOCK! A little old lady in a black dress, with Many Parcels, is in your seat!!! Do NOT confront her... a hassle will ensue. Just go find a deck officer to get her to move.