SpongeBob Squarepants: Revenge Of The Flying Dutchman Cheats

Jelly fishing gear costume:
In the Krusty Krab, body slam the target. A tent will become visible and you will have the jelly fishing gear costume.

Mermaid man costume:
In downtown (construction site), deliver Krabby Patties to Patrick and he will give it to you.

Reep blower costume:
In Jelly Fish Fields, win the 100 jelly fishing contest. Your prize will be the Reep Blower costume.