Sorry, I didn't check in yesterday! Stupid spring cold and all.
I remember there being one big pool that had access to the ocean, but also more seculsive swimming. I'm sure there are other pool, one has dolphins you can swim with (for a charge). I know the pool over at the Royal Waikaloan has a slide, but it's nothing like a water park.
Oooh,taking taxi out - you're looking at about a 35 minute ride, without traffic, at best. The traffic can get really stacked up on Hwy 19.
What you might want to do is find a copy of "Hawai'i The Big Island Revealed" and see what they have to say. The library should have a coy and you can find all the 'Revealed" books at your local bookstore. I'm thinking there's a waterpark closer to Kona now, but can't remember any of the details - a newer edition might list something.