Michisio Nagasumi is your average 14-year old schoolboy, who was dragged along by his parents to go on a summer vacation on the remote inland mountain beach of Seto. When he and his parents went to the beach, bad luck strikes when he got washed away on the sea, leaving him helpless on the fact that he is about to drown, when suddenly, while on the brink of death, a girl with a tail (in Nagasumi's blurred sight) saves him.

However, bad luck seems to be Nagasumi's brother-in-fate, when he discovers that the girl who saved him, named Seto San, is actually a mermaid. Catch is, San is the only daughter of Seto Gozabura, boss of the Seto Group, a Yakuza-like organization that has a grimly rule: Any human who discovers a mermaid/mermen must be killed, or the said mermaid/mermen would be killed instead. The only way for Nagasumi and San to escape this rule, is to get married (even if Gozabura objects, and only wishes to kill him instead)!

Facing dire (and stupendous) consequences, Nagasumi starts to embrace his eccentric new life, as he slowly adapts himself to the world of Mermaids and Mermen, as well as to learn more about them, especially his newly-wed wife, San-chan, while balancing them with normal Human life.

By: shinshinovi

One word: Humor.
You might also trip on some good values. Throw the bad values. and you get a good (but not total) package.
Seto no Hanayome is purely entertainment, and I can safely say I am entertained enough. However, critics might not agree, although I am more than sure they'll agree that Seto no Hanayome is clean fun.