I have one caveat before I write this. We went during the winter so we did not use any of the outdoor parks.
We've stayed at the four major waterparks in the Dells and Mt Olympus has the worst rooms of them all. The rooms at Hotel Rome are bare bones and dated. There are no drawers for storage, the sink is slow to drain. I don't like that the sink and vanity are separate from the shower and bathroom. The noise level is terrible. You can hear everything going on in the hallway, the room above you and the room next door. There is only one elevator by the main entrance, so be prepared to wait if you're checking in or out with luggage.
The indoor waterpark is decent. It has two tube slides, two body slides, lazy river and kids area. The indoor theme park is nothing special. It has a few kids rides, arcade and small go kart track.