First, I would drain the water. Then, I would grease the tub with some butter, so that it didn't hurt the hippo to get squeezed out of the tub. Then, I might gently coax it out with some candy, or a new book or video. If this didn't work, I would get together all of my friends to try to pull it out. If none of that worked, I might just have to break the tub with a hammer.