The one thing I wish I had was a pair of water shoes (I don't know what they're called, but you can wear them in the water).
We did a ton of walking at a huge water park with lots of hills...my feet were soooo sore at the end of the day.
Bring a blanket, towels and lunch and drinks in a cooler if allowed.
We hardly spent any time sitting, just a quick lunch break. We had to wait in lines and didn't want to waste the day sitting around.