If you're looking for manga similar to Seto no Hanayome, you might like these titles. All recommendations are made by Anime-Planet users like you!
Raku Ichijou may be the heir to a yakuza group, but he's a normal high school kid who dreams of peace and quiet. However, when he meets super-hot but violent transfer student Chitoge Kirisaki, his life takes a sharp turn for the worse!
Both of these school romances deal with a yakuza's daughter being suddenly forced into a relationship. The girl's family is so completely against it that they're constantly watching her new "lover," sometimes attempting to take his life in the process with hilarious results! To add to the list of people out for his life/against the relationship, there's a similarly overprotective assassin bodyguard yakuza in each too. Oh, and more than a single girl is smitten with the protagonist of course, one of whom's father belongs to the police - the natural enemy of the yakuza! While the plot in one versus the other is entirely different, if you liked what you've already read, you don't have anything to lose by giving another a try.