I don't believe that safety on the slides is based on swimming ability. I say this because there is no height restriction on the wave pool or lazy river, the only places you actually swim in the park, plus, some or the slides don't end in a pool, just a long run out of the slide, and they also have height restrictions. It has more to do with physics, height vs body mass, and staying in the slide properly. On some slides a small person that doesn't weigh enough might get stuck, or possibly get thrown out of the tube in turns. But can you imagine the uproar if they put a weight restriction on rides??

so they play the averages, and put a height restriction the ensures that everyone will have enough mass to safely use the slide. Of course there is a large margin for error built in, given the liability factor.
Last time we were there my youngest was 47.5", but they were very strict about not allowing him on, and for good reason. remember, they staff at the water parks are not just CM's, they are lifeguards, and can be held personally liable for knowingly allowing someone to ride that isn't tall enough, if they get hurt, so it's understandable that they are quite strict on the issue.