"My Wife is a Mermaid," is just the latest example of the strange girlfriend genre of anime. The basics are simple: typical japanese whimp suddenly gets a magical girlfriend, AND her friends, AND her rivals, AND her family. It might not be the first, but the prototypical example of this is "Those Annoying Aliens," aka "Lum" that alien in the tiny tiger skin bikini.
The title says it all here. Our hero accidently sees a mermaid and to save her life because mermaid law prohibits anyone seeing a mermaid he agrees to marry her. Oh, it should be mentioned these are YAKUZA mermaids, so daddy is a bit... extreme. Her mommy is all for it, and is like, well why aren't you already getting it on with my daughter, what's wrong with you. Our hero's parents don't care as long as all the damages are paid in gold coins. Oh, and there is a tiny girl in a shell who is homocidal, our hero's old girl friend (not girlfriend) who is the queen of school discipline, and the chief aide of the merman yakuza boss who is so beautiful he bewitches everyone regardless of their particular gender identity.
Oh and our mermaid grows her tail back anytime her legs get wet. So this gets into all sorts of "wacky" fun. It's not bad really, but I've seen it before better. I like the intro song in that it is done surf style rather than the usual idol singer style. Sort of fun, if you are an anime nut check it out!