The word is a compound of mere, mermaid/man, and naut.
Mere is the Old English word for sea. With the addition of “ma” (merma) both genders of mermaid and merman are represented. Finally, the term "naut" is from nautes (greek: ναύτης), meaning sailor.
(ALSO: sailing, boating, kayaking...etc.)
A: Because I am a Mermanaut.