The Cackling Cruiser is a large light blue fish wearing clown clothes, including a red nose and an orange wig (though it could possibly be his actual hair). He also drives a light brown boat with the number "8" on the side.
The Cackling Cruiser is first seen after Captain Lutefisk announces him. The Cackling Cruiser manages to ram SpongeBob, but SpongeBob rebounds back at him, causing him to crash into the wall and lose all of his tires, losing the Derby.
The word "cackling" means a shrill laughter, which fits his appearance and behavior.