For RV camping, campground B is the closest followed by E. For tent camping, C will be the closest followed by D. Water park as of last year only has a lazy river but it's good fun for few hours. There used to be a separate water fun area for younger kids but it was closed for renovations and might open again this year.

Sorry, I can't recall which are close to the water. You can get a better idea of the camping areas on the Casitas Camping Reservations website: https://reservations.casitaswater.org/reservation/camping/ We didn't visit the water park while we were there.

We just went, so I'm answering my own question. Stayed in the B campground which definitely was walking distance to the water park. The water park is small but fun. The lazy river was fun, and they now have 3 water slides and two big buckets that dump water. Saturdays are ridiculously crowded, but Sundays are much less people (at least this past weekend in August!).