My family will be departing Santorini (arrival on 5/4) on May 6 to ferry to Naxos. We will probably take the Blue Star ferry that departs at 3:30PM. Should I book tickets online now or purchase when we are in Santorini. Can the tickets be purchased in town or must they be bought at the port. We are staying in Firostefani.
I would just wait to buy tickets there, you can buy them in town (Fira) at any number of travel agencies advertising that line, at no additional cost. I usually would buy ferry tickets after I get settled and am wandering around, at least the day before departure.
You're early in the season and I concur you should purchase ferry tickets once you're on Santorini.
That ferry would never be full between Santorini and Naxos. Passengers on the other islands on that route back to Athens will be picked up along the way, meaning there has to be room for all of them as well as those departing from Santorini. You can safely wait until you're there to buy your tickets.