Nope. No way. Not gonna happen. DS will turn 9 next month, and is a good swimmer. I'm still not releasing him in the park by himself. Kids are kids. Even though he's a good swimmer, he sometimes doesn't know his own limitations, and there are just too many people at the waterparks. I'd rather be safe than sorry. As someone else pointed out, ds isn't even allowed to use the pool in our backyard unless someone is there to supervise him.
As for the idea that chance of someone being abducted or harmed in a water park being unlikely... well, that just isn't much comfort to a parent whose child happens to be the 1 in 50 million. Just because it's the Most Magical Place on Earth doesn't make it "The Most Crime Free Place on Earth". People do commit crimes on WDW property just like they do in other places. I can't protect my child from everything, but I can at least make sure that he's as supervised as possible.