What is a very simple painless way to become a mermaid?

The process of becoming a mermaid is not natural, and the only way known to modern man is probably through a series of difficult if not illegal surgeries. There is a birth defect where, in extremely rare cases, children are born with their legs fused together. This defect, called sirenomelia or "mermaid syndrome" is usually fatal within hours of birth. Obviously not a desirable thing, doctors usually attempt to surgically detach the legs, even though this may not guarantee the child will survive or be able to develop normally. Obviously, this condition gives no advantage to one's ability to swim or live under water as it is merely a cosmetic defect.

In fairy tales, most popularly, "The Little Mermaid", the process of switching from a mermaid to a human or back is magical and not widely practiced if at all even among those who would claim to practice magic. If you mate with a mermaid you can.