The Boat Jacker is a huge purple muscular fish that wears a ski mask and carries a crowbar.
There is another Boat Jacker mentioned in No Free Rides, when SpongeBob thought Mrs. Puff was one of them. It is possible that this Boat Jacker may be related to the reference SpongeBob made at Mrs. Puff.
The Jacker was in the SpongeBob Movie Video Game, only smaller, and in the levels Thug Tug and Dennis Strikes Back. The Jacker in the game, however, has a different voice, he grunts which are higher pitched than in the real movie.