Captain James P. Dirt was encountered by Tommy Gilligan when he was sucked down the drain of his bathtub.
History
He was apparently a child who refused to take a bath and has been running from his mother Mrs. Dirt since. Now a full grown man, Captain Dirt acts as the captain of the "S.S. Stinkypants", leading a crew of lazy and filthy children.
Captain Dirt's name and speech pattern are a reference to Captain James T. Kirk of Star Trek.
After Captain Jimmy and his crew rescued Tommy and brought him onto the ship, the two of them became great friends and subsequently fought together against Moby Duck, Captain Dirt's arch-nemesis who had followed him since he ran away from home.
After trying and failing to capture the giant duck, Captain Dirt, his crew, and Tommy discovered, to their shock, that Moby Duck was actually a giant robot piloted by Jimmy's Mom, who had chased him all the way out here just to force him to take a bath. Jimmy sadly accepted, telling Tommy (as his last words to the boy) to always listen to his mother.