We have been to their location in Wisconsin Dells many times. It is our favorite of the many water parks there.
Like others have mentioned, you can start swimming usually after 12:00 even though check in may be at 3:00 or 4:00. They will give you your wristbands and a pager so that they can let you know when the room is ready. Pack a separate bag with your suits so you can start taking advantage of the water park right away. There are locker rooms with changing facilities in or near the water park. On our first visit, we didn't know this so we didn't pack the extra bag. We waited around for our room which wasn't ready until 7:00 PM. We were furious so they upgraded us to an amazing 3 bedroom condo.
We usually bring cereal, plastic bowls, utensils and a half gallon of milk along with bananas, granola bars etc...so that we can have breakfast and snacks in the room. The food is overpriced and not that good. We bring bottles of water and snacks down to the water park in our beach bag. We've never had anyone tell us we couldn't even though the signs say no outside food or beverages.
They provide life vests which we always took advantage of. The lifeguard there are excellent, but you can't be too careful. The wave pool gets crazy when it's crowded. If your kids aren't strong swimmers, they should be wearing one.
If your kids are little, take advantage of the story time by the clock tower in the evening. It's so cute, and oh, the memories it will make. My 12 yr old wanted to go down for it the last time we were there just for old time's sake.
If you have the time, you can get an extra day at the water park by staying all day on the day you check out. You are allowed to do that. We usually check out of our room at 10:00 or 10:30 am, and pack that beach bag with our change of clothes. We stay at the water park until about 4:00, hit whatever fast food place is closest, then get on the road for the 3 hour trip home.
Enjoy! We really love GWL!