I would think it depends on the schedule. If your breakfast is at 8am and you go to the water park at 10, that wouldn't be a problem, IMO. If you're eating at 9:30am and rushing right over to get in the water the second the park opens, then you might feel a bit weighted down and queasy.
I wouldn't do it! I know how I feel after I eat at a buffet even if I don't pound a lot down. I would save it for a bigger lunch. When we go to the water parks and do all that running up and down to get on the attractions, we are always starving after 1 hr.-1 1/2 hrs. of play.
Well, I wouldn't want to be too active after a big buffet. But the whole don't swim for an hour after eating or you'll get cramps is a bunch of nonsense; thought everyone knew that by now.