Sorry if this is a silly question but how do Mermaid Man and Barnacle Boy breath underwater and not wear diving suits like Sandy does? i mean they look like humans do or is it that they are sea creatures that look like humans.

It's a cartoon. It doesen't have to be 100% accurate.

I'm not sure about Barnacle Boy, but look in Mermaid Man's name: Mermaidman.

They are actually fish dressed up as humans.

They're like a miniature (perhaps to do with the small ray) Aquaman and Aqualad and would have the same explanation.

They're superheros, they probably have the ability to breath underwater.