I've lived in the Jacksonville area for 10+ years, but we just discovered Shipwreck Island Waterpark this summer. We've traveled to Orlando and to Georgia to go to water parks and never realized that we had an amazing one practically in our own backyard.
We fell in love with Shipwreck Island - in fact, our family of four visited it 3 times this summer. My daughters are 6 and 8, and there was enough to keep them busy for an entire day. There are several water slides - some are a little twisty, while others are downright scary! There's also a lazy river, a giant pirate ship/kids play area, and a wave pool. My 8 year old loved riding all (but one) of the slides with her dad, while her sister and I spent most of our time in the wave pool. The water was really warm in the wave pool, and it was a nice, relaxing spot (except when the waves were crashing).
There are a couple of concession stands with a nice selection of food - or if you don't want to eat in the park, you can leave and re-enter. There's a Publix (grocery store) and quite a few restaurants within just a couple of miles.
If you're there with small children, I'd recommend getting there as early as possible - the park can get quite busy as the afternoon progresses. And if you are early enough, you can land a couple of lounge chairs in the shaded area next to the lazy river. There are also lots of umbrellas if you want shade, and lots of sunny spots if you prefer to be in the sun.
Adventure Landing also has an arcade and a "dry park" with attractions like go-carts, miniature golf, and a batting cage - but those are not included with a regular water park admission. Keep your eye out for coupons or combo specials.
We will definitely return to Shipwreck Island Waterpark next summer, perhaps even as season ticket holders. It's a great place for locals, as well as for families who are vacationing in the Jacksonville area.