Jungle treasure hunt movie. 80’s action?

Hello,

I’m looking for the name of a movie I saw years ago.
All I remember is:

Three protagonists(?) searching for treasure in the jungle.
It’s found at a Dam/reservoir/construction site.
At the end of the movie the protagonists have the boat they are escaping in shot up by the Bad Guy using an M60.
The boat sinks and the treasure is lost.

Any help would be appreciated, thank you.