We've been to Legoland quite a few times but this was our first trip to the water park. You actually enter the water park through Legoland. It's a $12 add on to the Legoland ticket if I remember correctly.
I had four girls 9,9,6,4 and they all loved it. It was very crowded the day we went. Weather has been very strange in Southern California this summer (coolest July in 77 years in San Diego) and this was the first hot day in about a week. I asked one of the employees if it was always so crowded, she said no the hot weather brought the crowds. It was actually too crowded as lines for the slides were 25-35 minutes long.
There are changing facilities, locker rentals, a food outlet, life jackets are free and plentiful (if you need one). They have cabanas that can be rented for full or half day. There is a little kid area with it's own lazy river and slides. Main area has lazy river with tubes that you can build legos on. I think there were three big slides, one of them was a multi-rider tube ride and I think the others were not.
I think you could do both in one visit but it's a lot to do in a day. I think a two day trip is a good idea if you can swing it.