Personally, I hate the water parks and my main reason is that if you go in the morning, you have time to get one (maybe two, if you're lucky) slides in before it's so packed. I refuse to wait more than 20 minutes for a 50 second water slide. Give me the resort pool anyday. That being said, (as I lay my curmudgeonly attitude aside) I think that you may be better going at 3:00. In general, I think the parks start to empty around that time, and from what I've read on the DIS, this seems to be the quiter time.
RyMickey