This would definitely be possible, at least during the slower times of the year - I guess the water parks sometimes hit capacity during the hotter months of the year. If you have a UPH, it would definitely be okay - if you don't, it might not be worth the price of admission for an hour or two.
We did exactly this on our last March trip - we went to the park in the morning - brought bathing suits in my pack, just hopped the bus to the water park, changed in the locker rooms there - rented a locker for the stuff, rented towels, and we were golden. We did go to Typhoon Lagoon though, as Blizzard Beach was closed.