The mermaid myth is a religious myth. It's all about the continuation of the soul.
Hans Christian Anderson. From Wikipedia
*The prince and princess celebrate their new marriage on a wedding ship, and the Little Mermaid's heart breaks. She thinks of all that she has sacrificed and of all the pain she has endured for the prince. She despairs, thinking of the death that awaits her, but before dawn, her sisters rise out of the water and bring her a knife that the Sea Witch has given them in exchange for their long, beautiful hair. If the Little Mermaid kills the prince and lets his blood drip on her feet, she will become a mermaid once more, all of her suffering will end, and she will live out her full life in the ocean with her family.
However, the Little Mermaid cannot bring herself to kill the sleeping prince lying with his new bride, and she throws the knife and herself off the ship into the water just as dawn breaks. Her body dissolves into foam, but instead of ceasing to exist, she feels the warm sun and discovers that she has turned into a luminous and ethereal earthbound spirit, a daughter of the air. As the Little Mermaid ascends into the atmosphere, she is greeted by other daughters who tell her she has become like them because she strove with all her heart to obtain an immortal soul. Because of her selflessness, she is given the chance to earn her own soul by doing good deeds to mankind for 300 years and will one day rise up into the Kingdom of God.*
A wee bit different from the Disney version.