The Creature Walks Among Us (1956)

In a dramatic scene from The Creature Walks Among Us, the creature, now with human lungs, escapes back into the water only to realize, too late, that he can no longer live in water and is drowning.

From Wikipedia:

The Creature Walks Among Us is the third and final installment of the Creature from the Black Lagoon horror film series from Universal Pictures, following 1955’s Revenge of the Creature. The film was released April 26, 1956, in the United States.

Following the Gill-man’s escape from Ocean Harbor, Florida, a team of scientists led by the deranged and cold-hearted Dr. William Barton capture him in the Everglades. During the capture, the creature is badly burned in a fire. While bandaging the Gill-man, the doctors notice that he is shedding his gills and even breathing using a kind of lung system. Now that the creature has more human-like skin, he is given clothing. The doctors attempt to get the Gill-man used to living among humans. Though his life is saved, he is apparently unhappy, staring despondently at the ocean. Barton ruins the plans when, in a murderous rage, he kills guide Jed Grant, who had made romantic advances toward his wife, Marcia. Realizing what he has done, Barton then tries to put the blame on the Gill-man. The Gill-man, witnessing the killing, and apparently comprehending that he is being blamed for the murder, goes on a rampage. After ripping down the confining electric fence, he kills Barton and then slowly walks back to the sea. He is last seen on a beach, advancing towards the ocean.