How feasible is it to do either water park and then hop over to another park for the evening?
Very! Both waterparks tend to close around 5PM for most of the year, so, picking up dinner and an evening park are perfectly reasonable.
You’ve gotta go somewhere with the waterpark closes, right?
It’s easy to do. You can leave your wet suits in a locker at whatever park you enter. Just pick a bus to the nearest resort to the park. Contemporary for MK, Beach Club or Boardwalk for EPCOT and Hollywood Studios, etc.
We’ve done a water park in the morning till lunch time, then headed over to a park for mid-afternoon with no problems. The lines at the water parks tend to get longer around noon, so for us it made sense to only do a few hours in the morning then use the rest of the day at a park.