Air seats versus Deck seats
 

I am trying to book a return trip Crete to Piraeus with ANEK line ferries; and on the outward journey there are Air Seats but on the return journey (it is a different ship) there are only Deck seats. Can anyone tell me the difference please as the cost is almost the same?

Air seat is reserved inside airline type reclining seat. Deck is just passage on the boat, no reserved seating, I believe.

Maybe all the seats are sold out except for general passage?

First of all, there is no such thing as return trip tickets on ferries — you are actually booking two one-way tickets. Secondly, there is more than one ferry between Crete and Piraeus, so choose one where you can reserve a seat. The air seats are much larger than the ones on airlines, with lots more leg room. You can get up and go to the restaurant or toilet without fear of losing a reserved seat. I would definitely want a reserved air seat (or even business class) for a trip as long as the one between Crete and Piraeus. :-)

Reserved is the way to go on such a long haul..Deck passage is not all that great on Greek ferries. If you can go reserved do it.