I've worn Tevas to both Disney water parks many times. Sometimes they will ask you to take them off and hold them on the slides. Then I point out that it is very hard for my feet to come out of the shoes and it is not a risk that they will fly off so they usually let them stay. I think the shoe removal request also has to do with reducing wear and tear on the slides, but a lot of times I'm able to politely convince the CM to let me keep them on. I never cause a scene or anything, it's just that once I get them tightened the right way it is hard to get them like that again. :shrug:
i agree with epcotWSC, any type of footwear is good, but i would also throw in the use of dive booties, which are much more secure (thanks to the zipper on the sides). of course like any footwear, you'll have to remove them on most body slides.
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