Often located at the highest point of the lagoon's rock face. Red Key/Switch:

Navigating the Depths: The Ultimate Walkthrough for Shark Lagoon Welcome back, adventurers! If you’ve reached the Shark Lagoon The Jungle Call

Note: If you were looking for the secret "code" (sometimes used in older Shark Lagoon versions to unlock the game), it is typically "shark" or related to the game title, entered in the code box on the main menu.