Creature From the Black Lagoon
(1954)

A great monster movie produced in 3-D.

The expedition to the Black Lagoon finally captures the gill man, but he escapes and comes back to abduct Kay (Julie Adams).  The adventurers rescue her then stab and shoot the gill man, who is last seen sinking to the bottom of the Black Lagoon.

Not to worry, he comes back in Revenge of the Creature (Eastwood's first picture) and The Creature Walks Among Us.

Thanks Don P!