Does anyone know of an 80′s or 90′s movie?

I think its a comedy movie, its about some young guys that are looking for treasure under the ocean. There’s also an old guy that is with them that wears a dark coat and sunglasses and he’s kind of cookoo, but he’s funny. Towards the end of the movie the cookoo funny guy goes into the ocean and walks underwater after the guys find the treasure.
Please help! I can’t stop thinking or looking for this movie!!

Weekend at Bernie’s.

The guy wearing the dark coat and glasses is Bernie, and he is dead.

Underwater Treasure Hunting