A man in a white and gray shirt looks on as a shirtless man with a mustache cleans fish on a pale green table.
The man watches the other clean a fish.
The man watches because he looks on.
If a man in a white and gray shirt looks on another man who is shirtless and cleans fish, it implies that the man in the white and gray shirt is buying filleted fish from the other man.
For a man to watch the other he would have to be looking.