Last time we went it was early May. It was high 80s low 90s the whole time. We usually went back to the resort to swim mid day and then returned to the park in the evening, except for a few days where we stayed all day long. I think I might have put on a jacket one morning, but that was after we had been there for over a week and I think my body just adjusted to the warmth so the 65* weather that morning was just too cold.

My family however made fun of me that morning, because they were all in tank tops and shorts. But they did regret it when we had to wait in the shade to get into the park for our dining reservations. Then the younger girls were cold.
Each night was warm enough for the kids to swim though, so it didn't really get to cold at night. On our last day it got so hot though I think it was 97*, we really went through the water that day. It may have felt hotter than it was because we were in Epcot WS that day, and it always seems hot then.
Last April over Easter break we went to visit my mom and went to Sea world and Aquatica for a few days and it was really warm then too. But it wasn't so hot you didn't want to be in a park.