There are two major water parks in Dubai: Aquaventure and Wild Wadi. We visited both. Aquaventure is located near the luxury hotel Atlantis on the Palm island. It's better than the Wild Wadi water park.
You have to pay for lockers ($11) and towels ($8). That's why we took towels from our hotel and took only one locker.
The water park is outdoor, as all water parks in the Emirates. It is very cozy, with lots of greenery. You can float on the "river" all day long, up and down, using the lifts to climb the hills.
The slides are not very scary, but fun. The most amazing one is where you end up in the glass tube located in the tank under the water with stingrays and sharks. The scariest waterslide is the yellow one which starts at the top of the pyramid. But we were afraid to ride it :)
If you come to the water park, we also recommend visiting the Lost Chambers Aquarium. It is located right on the ground floor of the Atlantis hotel. Aquaventure visitors can also have a rest on a huge beach on the last branch of the Palm island.
Prices: Waterpark - 225 dirhams ($60, for children under 1 m 20 cm the price is lower), Aquapark + Lost Chambers Aquarium - 275 dirhams ($75), check out the prices at the website.
Getting there: There are only two ways - by monorail and by car (taxi). The monorail goes through the entire Palm island and it is interesting. But to get to the the monorail in any case you have to take a taxi from the Dubai Internet City Metro Station 2 or Nakheel Metro Station 2, as we did.