Jessica, Jessica, I know you have started many threads, but just as a reminder -- when you start a new one, it's useful to recap the basic information-- not everybody is familiar with your past postings. the Dates of your trip are key. I hunted down another of your threads and found that your trp is June 16-July 17. Therefore you do NOT need to buy tickets before you arrive in Greece.I believe u are flying to Crete & will be there 4-5 days. On Crete, you can buy the 9:00 Hellenic Seaways Highspeed tickets to Santorini -- These are large car-carrying catamarans ... not sick-making unless (unlikely) there's a storm. The other Ferry line has smaller vessels. You do NOT need to buy them at/near the port -- any agency in Chania will do. Same with Santorini -- you can buy tickets in Oia, Fira Town, beach towns, etc. In Naxos, you are bound to be in the port town your first night or next day -- so much to see there (like the Portara!)
When u get to Santorini, decide if you want to sail to Naxos in the AM (10:30 or so) or PM (about 3:15-3:30). Morning, the Champion Jet (500+ seats) has outdoor deck at back for views on departure -- it's a fast boat, expensive. Afternoon is BLUE STAR (1500+ passengers, 250 vehicles) is very large, more like a small liner ... plenty of room . Get economy (deck) seats for the 2 hour journey. After Naxos, BLUE STAR runs by day 9:45 - 3pm, or evening 6 - 11:30-11:45. Long trip but you can get "reserved economy" seats indoors... leanback, upholstered comfy. Again, plenty of time to buy on Naxos ... only thing to watch is, if you're going to Athens on a SUNDAY, both saiilngs fill up, so think ahead.