The Tuanjiehu water park rocks!

Right in the heart of the city. Generally clean and spiffed up for 2010 (though now its closed for renovations again for the fall/winter).

My 3-year-old loves it, though some of the features of it are really for older kids (and plenty of adults hang out here as well).

here's some tips for when you go
-- They don't sell suntan oil/sunblock, so bring your own
-- Plenty of swimwear/towels/flotation devices for sale
-- Food is for sale at the park, but kinda gross (generally BBQ skewers that might have been frozen for years and things like pre-packaged oreos)
-- Not supposed to bring food in, but they don't check your bag
-- there are safety lockers for valuables available for 20 kuai each
-- there is a small beach area. bring a small tent or beach blanket to stake your area.
-- The adults smoke like crazy, and no one stops them
-- Older teens and young adults love to roughhouse (splashing fights, wrestling, etc) so watch out a bit
-- Dressing rooms are pretty foul, good idea to wear your swimsuit to the park to save at least one trip

The Tuanjiehu water park is, by American standards, not very nice at all. The place is clean--even the locker rooms--and there is a knifty locker system to keep your valuables safe. However, the actual entertainment portion is lacking. What's billed as a tide pool is basically a sheet of water coming out of a random wall in one of the pools, and there are so many Chinese people gathered around it that you're constantly knocking into people. The water slides are ok. There's one straight one and a more complicated curling one. If you really launch yourself you can get up to decent speeds, but the landing pool on the straight one is too shallow (always hit my back on the bottom) and the curling one can launch you dangerously upwards towards the bottom. I think this place would be a lot of fun for little kids, as there were a lot of shallow pools and places for hanging out, but if you're looking for a six-flags level experience this is not your place.