I would be happy with a stuff pack. I don't think what the OP posted along with things like pool floats for various age levels, pool games, and a pool bar is too much for a stuff pack. Maybe a GP?
I'm having a hard time wrapping my head around a full Waterpark EP just because I'm not sure how that would work in a closed world. I suppose the world map could be set like a park map with several very large lots that can be edited with different rides.
I think at one point there was a survey that included something along the lines of a Waterpark. Anyone remember that?