I sent this question to disney (via email) a few months ago:
I would like to know what the rules are for wearing watersocks/sandals on slides at the water parks. I have "very" sensitive feet and would like to wear my water sandals on all slides/attractions and want to know what the rules are.
Thanks for your response.
Their response follows:
You are very welcome to wear water sandals on the slides, provided they do not have metal buckles or rivets that could damage the slide. Most mesh-style rubber-soled sandals would be entirely appropriate, and even advisable, from a
safety perspective.
So the answer is yes, you can wear water shoes or sandals on the slides.