My girls hang out in the tub too. After I get out of the shower Iris goes in there and plops down to watch the water drip down the walls. She come out about 15 minutes later with a wet butt and legs.
Sherman is a nut for the bathtub! He paws against the sides then goes sliding around for awhile.
Both of my cats are petrified of the shower. Once the bathroom door has opened one of them will slink in as quiet as possible and crouch by it watching the water drop. They're so curious - I think they're trying to figure out what it does.