In mid-July 6 of us are flying into Heraklion, Crete from Athens and staying for 3 days. We intend to take the ferry to Santorini from Heraklion as walk on passengers. Is it necessary to book and buy ferry tickets ahead of time on line from the U.S. or can we wait and buy tickets while we're staying in Heraklion ??
You can buy them in Heraklion. It's a good idea to wait until then because the seas get rough in July and sometimes that particular ferry is cancelled. You will be able to determine once you're there whether or not you'll have to make other plans.
Lee's right about weather issues. On the other hand, we took that exact ferry a few years ago and it was completely sold out. We were very glad that we'd bought our tickets in advance. The Flying Cat and the Sea Jet catamarans each make only one trip daily, and the Sea Jet only runs certain days. (We took the Flying Cat.)
Tom's point about those ferries being sold out is accurate. I should have thought about that. It would be a good idea to purchase your tickets in advance. Hopefully you won't have any weather issues. To recuce the possibility of seasickness sit along the center line of the ship minimize the side-to-side rocking motion.
My wife and I purchased our tickets the day before we traveled and we had no problems. If you can afford it, take the Flying Cat 4 hydrofoil instead of the regular ferry. It was fun and much faster for just a little bit more money. When you get to Santorini, visit the town of Kamari on the south side of the island. It is a town where locals live and not nearly so many tourists.
Ticket prices on the FC4 are approximately double what they are on a conventional car ferry. The bad news is that the car ferry is infrequent at best. Kamari Beach and Perissa Beach are the two most heavily-trafficked beaches on the island and in July those beach hotels will be overcrowded with package tours from the UK and Germany.