You found it, Sly! This is where Raleigh's gang of crabs stashes his sunken treasure.
― Bentley to Sly, as the latter enters the crabs' lair[src]
Sly hopped inside a submarine and dived down into an underwater room, where Raleigh's gang of crabs stashed his sunken treasure. Sly had to use the submarine to destroy forty treasure chests in order to gain the treasure key, doing whatever it would take to prevent any crab from making off with any treasure.