We went in April (9th-23rd), it was lovely, I found it a bit cool in the mornings, EM hours 8am so I wore a hoodie. Apart from that we were in shorts and vest tops the whole time. We had a morning at Blizzard Beach and had a great time, it was just a but cool waiting in line to go on a slide. The last 4 days we spent at St. Petes Beach it was gorgeous, fantastic sun bathing weather.
My trip report is on here from April 2007, quite a few photos, it will give you an idea of the weather by what people are wearing.
Sharon.