Mermaid's Scar(1993)

Ningyo no Kizu

Legend has it that when you eat the flesh of a mermaid, you would live forever. Yuta and Mana are living proof of this... Together, they journey to various places trying to find meaning for their existence, or perhaps even a "cure" for their situation. One day, Yuta and Mana meet Masato, a little boy who seems terrified of his mother Misa. It seems to Yuta and Mana that mother and son have a very unusual relationship. What happens when Yuta and Mana decide to discover the dark secret Masato and Misa are hiding? And what about those monster-like creatures that have been appearing?

(Source: AniDB)