I love water parks and I know Vegas has a couple of them. My favorite is hands down the Cowabunga Bay water park. Ever time I go to Vegas, I stop there for a while. Especially in Vegas, it feels like the best place for a water park.
Cowabunga Bay is a pretty good place to take the family. The water park is in Henderson which is maybe a 15-20 minute drive from the Strip. If you do go during the summer (or really any day for that matter) bring plenty of sunscreen. We had family members from out of town go once and they came back from the pool with a deep red sunburn.
I have only been here once and to another water park called Wet'n'Wild. Of the two, I would recommend Cowabunga. I am not much of a water park person though so for me, it was more about just cooling off than going on the slides and everything.