We chose to buy the second day for the waterpark. There are 2 large issues with the waterpark. First, the water park is at the rear of the main legoland property. If you are just going to the waterpark for the day, you must trudge all the way back of Legoland in the heat. No transportation is available and no separate entrance. Second, you must endure the waiting in line a second time to enter the park because they have a second ticket line for entrance into the waterpark from Legoland.
The waterpark is ultimately designed for small children. Consult the website. The park is basic. The best part for kids is playing with the foam lego pieces in the lazy river. If you have larger children (10+) they will most likely benefit from a larger waterpark.