Splash (1984)

Ending / spoiler

The government finds out that Madison is a mermaid,and she is hauled off to live in a lab tank for studies.Her boyfriend Allen and his brother Freddie(both humans),discuise themselves as scientists and sneak Madison out of the lab. Madison and Allen then go to the ocean,thinking about either releasing her back to her ocean life or stay a human and live with him,but they must make a fast decision because they are surrounded by the people who captured Madison.Allen then jumps into the ocean with Madison,and they swim around down there together,choosing to stay together forever.