I have worn my soft contacts to the water park. As another poster said, just don't open your eyes underwater. I generally keep my head above water, so there's not too much chance of my contacts being contaminated (and I don't worry about them "washing out."). If you're worried about them getting contaminated, you could bring an extra pair and throw them away after your day at the water park (of course, this would only work if you wear inexpensive contact, like me! Mine cost about $25 per year from Costco!).
Regarding water shoes and flip flops, I don't wear either around the water park. I wear my flip flops from my resort to the water park, then leave them at my beach chair. I was worried about hot pavement or rough pavement hurting my feet when walking around, but this wasn't an issue at all. And water shoes are such a pain - I even read that the rubbery material can virtually "stop" you on a slide, so you have to take them off when on a slide. What a pain! I thought it was fine to walk around barefoot.
Have fun!