Seto no Hanayome

Nagasumi visits his grandmother near the sea during the summer. While visiting, he goes for a swim, and drowns right after entering the water. Fortunately for him he is saved by a young mermaid. Later as he tells the story to his family, a young girl named Sun appears at his home and begs Nagasumi to marry her.

She’s followed by two large thugs and they take both him and his parents to Sun's home under the sea. If that wasn’t strange enough, it turns out Sun's parents are mob bosses. Nagasumi is left with the choice of either death or marrying Sun. He chooses to marry a mermaid.

This would likely be that, except Sun's father and some members of the mob are not exactly thrilled to have Nagasumi courting Sun and are determined to bring an end to their relationship - or him. And there is also the matter of keeping it a secret that Sun is a mermaid.

Source: anidb