Complex sentence: As the confused skipper tries to figure out what it was, another noise is heard in the boathouse (This noise is louder) As the walls start to shake, the skipper realizes that the shark is on the outside trying to break in.
Split into simpler sentences: As the confused skipper tries to figure out what it was, another noise and the walls start to shake. The skipper realizes that the shark is on the outside trying to break in.