If I get a room for a night and I check out the next day and I want to go to the water park again, if I get wet we're do shower or change?

After you check out, returning to the Waterpark isn't an option, but there are showers, restrooms, and lockers in the park.

You have access to the waterpark until closing on the day ypu check out. And yes, there is a locker room near the entrance to the park with showers, with shampoo and body wash provided.

There are locker rooms with showers in the water park area.

If I remember right there is a seperate showers & locker rooms to the left of the Main Waterpark Entrance to do so.