Ugh. Obviously Mermaidman was a mermaid bitten by a radioactive man, gaining the powers of two legs, and barnacle boy was a barnacle magically chimera-ed with a boy FMA style.

Media literacy is just dead, and everyone needs everything explained to them.

My headcannon was always that they were action figures. The episode where they all go on the surface shows all of the characters as puppets !

so in my head all of the characters are real items that are being animated by some sort of undersea magic.