If you were to do just an hour or so you'd be fine. And if you go right after a giant breakfast you'll be pretty good.
The problem that we have is that all that activity is just unpredictable for BGs. My son, like most kids, just drop like a rock as soon as they hit the water. I'm assuming you're still on shots, and the issue is deciding the day before, or that morning, how much less lantus (or levemir) you're going to give. And if you guess wrong, you can't take it back. You're stuck with a body full of insulin and a kid who doesn't need it. If you guess too little, you're going to end up with ketones later that night. On a pump, we worry about being disconnected for too long and that ketones develop. We worry about highs later on, deciding how much missed basal to give. It's just a complete cluster. Our kids like going to the water parks, but even with two parents to run and get supplies if we need them, it's still stressful.
I totally feel your pain though. My son was just invited to a water park with a school friend. He's almost 14 and I doubt this other parent has any clue my son even has diabetes. But it's just not something that he can safely do without me being there. It's really one of the few things we've come across, so that's really not all that bad, all things considered.