It lives close to water. Its long tail is ridged with a fin, which is often mistaken for a mermaid’s.
Its cell composition is similar to water molecules. As a result, it can’t be seen when it melts away into water.
Tests show that its cells closely resemble water molecules, which perhaps explains its ability to conceal its form while submerged. I believe the origins of mermaid folklore lie with this Pokémon.