The Mechanical Sea Monster is a contraption invented by Robert. It is made to scare off intruders.
When the Pirate trio arrived at Robert's hideout, everything was safe. But then Elliot saw bubbles blowing from the surface. Terrified about the creature that was attack his friends, he decides to save them by slaying the beast. But instead, gets eaten and George and Sedgewick were scared of what happened. Fortunately, the monster suddenly stopped and Elliot cuts through the neck, and tells them that it's mechanical and he shut it down by pulling the lever.
The Sea Monster only appeared in The Pirates Who Don't Do Anything.