I feel I wrote this review a bit messy and relied too much on the reader actually having seen this anime. If you think so too and don't follow with stuff I have written, please leave me a comment. :)
Story: A boy named Nagasumi nearly drowns in sea. Thankfully his life is saved by a mermaid named San. However due to some odd reason, he is now forced to marry said mermaid. She also happens to be the only daughter of a mermaid yakuza boss. The basic concept for the story sounds somewhat cliche. There are tons of anime with similar story but with aliens or other supernatural beings in place of the mermaid yakuza. The good thing about Seto no Hanayome is the fact that it is a really crazy comedy, because of that the poor story doesn't matter much. This anime doesn't start great and it doesn't end great either, but the middle episodes are truly hilarious. I almost dropped the series around episode 4. Although the first episode was funny, episodes 2-3 were not. They kept on repeating the same joke, namely the part where someone wants to kill Nagasumi. Episode 4 seemed like a prelude to even a weaker plot. In that episode there are hints of the show turning into a harem one. Thankfully the harem part doesn't get developed for another few episodes, and when it does, it never gets really important (it is mostly used for gags). Although the jokes are good through most of the series (especially episode 11), it seems they started to run out of good ideas towards the end: the ammount of fanservice had increased, they constructed two separate episodes around similar idea (episode 19 and 21), the plot of episode 24 was extremely generic. As I mentioned before, the ending isn't very good. They tried to do something serious in episode 25 and 26, but it felt forced and dull. By the way, I wonder why did episode 20 and 21 felt so wrong...? ;P
Animation: The animation was generally good, but some cheap tricks were used from time to time. Since those animation tricks were painfully obvious I can only give 7.5 points for animation instead of 8.
Sound: The OP song was rather good. I don't remember any of the BGMs, so it means they weren't bad, but weren't great either. On the second though I do remember some of the BGMs. There were a few good musical references among them such as Bach, Wagner, Phantom of the Opera and main theme from Terminator. Also, Some of the insert songs were quite nice. The seyuu did good job with switching between characters' various personalities (as in: one character = multiple personalities).
Characters: Nagasumi was hilarious most of the time (especially his many different faces). San was ok, as she helped with the gags and even added a bit of romance. Mawari looked and behaved strangely similar to Haruhi (the melancholic one). The yakuza members were really funny (Shark Fujishiro, Masa and Co.), though the yakuza boss was sometimes annoying (especially when he abused Nagasumi too much). Speaking of annoying characters, I didn't like Maki and Luna. I'm not particularly familiar with the yondere character type, but if Luna can by classified as such, then I guess there is a character type I like even less than tsundere. The good part about Luna was her father. He was so ridiculous that one could not help but laugh whenever he appeared (and they even played a proper theme whenever he did). I got mixed feelings about Nagasumi's parents. Their lack of care for Nagasumi was funny, but kind of sad.
Overall: If you enjoy watching really crazy comedies (like Hayate no Gotoku), then odds are you might like Seto no Hanayome, but if you hope to see some decent story or romance, you'll be disappointed. It's the craziness I liked Seto no Hanayome for. As I mentioned in the story section, this anime doesn't start great, but gets better after a few episodes, so it might be worth watching more than 4 episodes before deciding to drop it.