Waterparks are a MUST for any trip during the hot months. We were there 2 weeks ago and went 3 times to the water parks. We went first thing in the morning for the EMH, went on a bunch of slides and swam with the sharks at TL in the morning, then floated around the lazy river for a while, went back to our lounge chairs then had a quick nap during the busier afternoon. After we woke up we'd head back to the resort and shower up and go out to another park. Waterpark days were the best days for us becaue we weren't worn out from the heat in the evenings.
If you think you'd go to the waterparks more than once (or even to DisneyQuest) then it's worth adding the waterpark and more option onto your ticket. Admission is pretty pricey, like $36/person or something.
If you're only going once then it'd make more sense financially to pay at the gates. If you're only going to 1 park i'd reccomend BB. The kids area looks like a lot more fun and the wave-pool is managable. (The wave pool at TL has 6 foot waves!)
Have fun on your trip and defnitely check out the waterparks! Oh, and if you do want to swim with the sharks, go first thing in the morning before the crowds get too busy. That way there's no one bumping into you and you can take your time as you float from one side of the tank to the other.