We went in the month of May 2006, the water is cold. I opted for the wetsuit, it wasn't too bad. Hubby used the vest, was a little chilly!
Have fun, it was one of the best experiences we ever had.
Another great Dolphin encounter is the Dophins in Depth tour at EPCOT! It is wonderful! We have done both. If you want a great day at a tropical water Park with lots of encounters with water life and birds, Discovery Cove is the way to go, if you just want an experience with Dophins and don't care much for spending the rest of a day at the park, the Dolphins In Depth Tour is AWESOME!
ariel