We did this at both BB and TL on our last trip(in June) and it worked out great. We arrived around 1pm. It was crowded when we arrived, and the crowds gradually went down throughout the afternoon. For the last hour the park was open, we were going on slides over and over with no lines at all. But there is one big problem with this strategy. Afternoon thunderstorms. It's very common to have thunderstorms mid afternoon in Orlando in the warmer months. And if there is lightning, the water parks shut down. We got lucky both times because the thunderstorms didn't start rolling in until right about park closing time. But you are definitely taking a chance of having your water park afternoon ruined by the weather.