Maybe the Soaked expansion for RollerCoaster Tycoon: 3 (you'll need the original game) holds a clue.
In it, you build monstrous body slides, over-sized swimming pools and cartoon log-flumes for the paying visitors to your expensive theme parks.
Water, in Soaked, is fluid cash. Every guest to your new swimming pool pays an entry fee - for which they're granted access to the pool itself, spas, sun-loungers and any chutes or slides you might bolt on to the sides.
The latter are stupidly good fun to build, but watch out - our first attempts ended up catapulting victims over the landing pool, off the edge of the tiles, and down a 200-foot cliff. Hilarious, but not good for business.
Building a successful water park requires a keen eye for space. Fitting in all those slides and attractions can be difficult, but your heart melts when the first customer leaps from the top of the diving board - with a gleeful "whee!" - right into your huge pond.
Maybe that explains why all those towns needed water parks - they're guaranteed to lift your mood.