I feel like most of the comments here involve way more content than we ever received in a stuff pack, even the later ones. I think we should either scale up @JAL 's idea and have full on Water Park Game Pack with a water park world with a usable monorail or other transit system for traveling around the park, as well as semi active careers for sims who want to work at the park. Maybe a hotel for taking a water park vacation, as well as all of the other things you five have suggested.
Or we could scale down @Mak27 's idea and have a stuff pack with a pool party and maybe Marco Polo/Sharks and Minnows, lots of swimwear with floaties as accessories, and various pool objects.