Forgot to say.
At certain times of the year the theme park is open to midnight.
We found going in the day too hot, but going in the evening was much nicer, and most of the crowds had gone by then so the queues were shorter.
They also have an amazing show, with fireworks, most nights at midnight.
The theme park site is huge, here is a picture from the air (you can see the water park in the bottom left hand corner)
You can easly spend 2 or more days in the theme park and still not see everything.
http://www.umwebsite.com/6images/aereas-um-01. jpg