There is wet and wild up near universal studios and then there are two Disney Water Parks - Blizzard Beach and Typhoon Lagoon.
I believe Busch Gardens also has a water park next to it, but its a long journey for just a water Park.
You can by annual passes for Wet n'wild and also for the Diney water Parks, this could work out cheaper if they plan on visiting a few times. You would need to look at the parks own website to find the prices for these.
The only thing to watch out for is that you are going to hit the sumer months when you often get a quick thunderstorm in the afternoon. If this happens guests have to clear all open areas, so it may be best to go early morning for a few hours then go off and do some shopping or something. This would work well if they are going to visit in several occassions anyway.