You hit the nail on the head. Local water parks have to add things to get locals to come back. Disney is not trying to get locals with their water parks, they are trying to get their guests to add another day to their vacation by tacking on a water park. Remember a day added is hotel, food, spending, etc in addition to the price of the water park. Disney doesn't need to add anything to do that.
BTW, is the water park in West Palm any good? I have never heard of it, so that tells you how little they advertise to vacationers. We always say there is nothing to do in WPB and we go to relax

Might have to visit the water park if it is any good for toddlers, my daughter loves water