Lake Compounce in Bristol, CT, has Bear Creek Campground associated with it, and a water park area (that is new or significantly expanded within the last year or two, if memory serves). Knoebels, in Elysburg, PA, has a very nice big pool with a couple of water slides, too, and one of their campgrounds is right next to the park (and on the pool side). It's a decent drive from southern New Hampshire.
Button Bay State Park, in Ferrisburgh, VT, has a fairly nice little pool with a slide etc., but it's certainly not a water park by any stretch. I'm sure there are plenty of other campgrounds with nice pools; it's just the one I'm acquainted with offhand.
