A group of friends is travelling to Greece soon and wants to take a ferry from Piraeus, Athens to the island of Naxos.
They've tried to book tickets through http://greeceferries.com/ (which uses Seretis Travel) but it appears that they only allow you to get tickets for a cabin -- you can either book the whole cabin or split by gender into separate cabins. They're a mixed group so this is undesirable, plus they want to be out on the deck anyways (and it's cheaper).
Is there anywhere online where they can book deck tickets? Has anyone else had experience with this or other nearby ferry trips? Is it easy to get tickets on location if they can't book online, or will they sell out days beforehand? Any other concerns?