Harem anime aren't known for pushing the envelope and trying new things - don't expect anything shocking here. Nagasumi (a run-of-the-mill, white-button-down-shirt-wearing, soon-to-be-pimpin' boy) is saved from drowning by a mermaid (San) - but any human who finds out about mermaids is killed, along with the mermaid. But wait! There's a loophole! If they get married (and join the mermaid family) then they're spared. What's more, the mermaid are all Yakuza thugs who rather liked the idea of eating a landlubbber. Cue the wackiness waiting in the wings.
Arranged-marriage hijinks are a prominent sub-strain of harem anime (despite generally focusing on one girl, they still remain in the family by keeping with the twin pillars of Nothing Ever Happens and There Are Always More Hot Girls, even if they're just toss-away) and Seto no Hanayome doesn't break any stereotypes, and it suffers from a core romance with little actual chemistry. There is hope, however, that Japan have finally realized how to revitalize the boring and indulgent genre: have a male lead the audience doesn't hate more than they envy.
Nagasumi a pretty nice guy and he does a stand-up job of acting like a dude a chick might actually care about, but more importantly he's also a complete nut and the animators go hog-wild with his reactions. He's constantly swinging from dopey lovey-dovey to scared-to-death to completely dead at the drop of a hat. He pinballs around the cast, bouncing off each character with increasingly colorful reactions, and the show follows suit with wildly divergent animation styles. The fact that his bride's family are all over-protective Yakuza more than ready to chew the shit out of the scenery provides Nagasumi more than enough to freak out about.
The humor and variety of animation keeps the show fresh and fun, even when you want to write it off. Recommended for anyone looking for an amusing and kinetic romance. Highly recommend for anyone with a soft-spot for cute girls speaking with a Yakuza snarl. Like me. Don't bother looking for deep thought or romance or anything beyond the basic "high school kid gets engaged to a Yakuza mermaid" joke.