J. - the two waterparks here in Lewisville/Flower Mound are the city-maintained pools/parks - one is behind Lewisville High School, and the other is on College Street. Both of these are right by I-35 off of Main St. They are cheap - $2-4, 2 & under free. Big difference between the parks, though - I am a big frequenter of both (i have a 2 1/2 year old), and here are the main things of each:
High School park: the bigger one. Has a big lazy river, a big separate diving/olympic pool, big water slide. Very fun for the older kids. Also has a separate tot pool - big round thing that is only 6 inches deep, with own lifegaurd. The park is bigger and crowded - one of those places that parents might drop kids off for the afternoon, meaning lots of bigger kids running around, etc. The tot pool is awesome, and the lazy river is nice.
College park: Very small. You can sit anywhere and truly see the entire park, so great for keeping tabs on kids. This is geared to the smaller set - 5 and under - no separate tot pool, several little water fountain splash - run through areas, a little bitty lazy river, one big slide. At this park, most parents are in the water with their kids (because of age), and it is way less crowded. The main pool has a "beach" type thing - meaning it slopes down to become deeper so you can sit up at the top part and splash around with babies while the little bit older can venture a few yards down. (the other park has this, too - but due to the older kids, harder to hang out there).
I prefer the smaller park, but that is due to my son's age and my neurotic fears about him getting hurt. :) If you go to the city website, you can find info about it - www.cityoflewisville.com - then scroll down on the main page until you see the headline/blurb about the water parks being open - then you can click on that link for all the info.
The only other water parks I know of (besides the big expensive Hurricane Harbor) would be Hawaiian Falls over in the Colony (I think), and then maybe Lifetime Fitness there on Cross Timbers - I would think you could get a day pass to check it out.
Have fun!
S.