It's going to be hard for someone else to answer that for you. It really depends on where you're from and what your cold tolerance is. I know when we lived in Florida, we used to go to the beach in our coats in January and you could count on seeing people from up north swimming. No way in the world you would catch me in that water, but they were enjoying themselves.
That said, on our October trip a few years ago, we did the water parks and it was tolerable. I think the day we went it was in the upper 70s, maybe 80. It was cloudy and cool in the morning, so it was chilly then. It got better as the day went on. Keep in mind that even if the water is heated (which you couldn't prove by me), you still have to walk from 'ride' to 'ride' while you were wet, so if it's not warm that alone can be chilly.
Jeff