There is really no advantage to pre-booking your ferry. May ferries will not be sold-out.

I would recommend waiting, as you can see all of your options. On-site, you will see a number of ferryboats departing Santorini that you would never see if you limit yourself exclusively to those listed on the Internet.

Remember not all ferries sail daily. Schedules can be kind of intermittent...i.e. some may not sail every third Tuesday. Also note to check with a couple of the harbor sales offices (or travel agencies in Fira or Oia) to fully access your options. Some ferryboats are not listed with every agency.

There is a high-speed ferry (5 hours) in May operated by The Hellenic Seaways company. I seem to remember it goes via Naxos/Paros, and departs early in the AM. It looks like the schedule is not out yet, but I would suggest you watch their website. You can pre-book online.

http://www.hellenicseaways.gr/index.asp?a_id=209