_Seto no Hanayome_ stars the generic harem comedy male lead whose main characteristics are mysteriously attracting female attention, remarkable physical endurance, and the devil's own luck. This scenario features him being saved by a mermaid who then has to marry him to prevent him from being executed for knowing about mermaids; subsequently, he 'enjoys' the attention of her unhappy relatives & yakuza loyalists, admirers, and rivals, who all accurately enough observe that she's too good for him, but of course it turns out to be true love anyway. (A time-honored formula going back at least to _Urusei Yatsura_.)
The plot is heavy on cliche. Each character has their shtick and boy do they ever stick to it (don't expect characters like Gozaburo Seto to change through the many episodes; Shark-san will continue to try to eat the lead, the mother will swoon over Masa, etc etc).
What rescues SnH from the delete-immediately category is its willingness to take the 'secret mermaid society' conceit and run with it (the mermaids' inborn fear of cats; San's fear of her policewoman classmate Mawari; the Seto Special Squad brothers; the Seto TV shopping channel with tail-enhancing prosthetics; Nakajima constantly being an octopus without it being remarked, and octopus kebabs showing up suspiciously often), and to engage in over the top parodies (the two-episode war between San & Luna loyalists, chockful of _Fist of the North Star_ references & shonen parody; the similar OVA 1; episode 24 where Kai is dying; the Masa mini-episodes slyly mimicking the 'bar scenes' in yakuza/crime dramas, where the game is to guess which character he's talking to before the punchline; Lunar's Papa's _Terminator_ episode and then later allusions; Saru's ero-hermit; episode 19's jidaigeki setting & equation of man-mermaid pairings as a threat to the existing order akin to pro-Imperial revolutionaries in Tokugawa Japan; ep22's depiction of San's ideal delinquent boyfriend). The more 'serious' romantic arc like ep02 and ep25-26 are also good enough to not leave a bad taste.
What stops SnH from being better than the watch category is its many wasted opportunities.
For example, the tail-changing-in-water mechanic might be considered to be the prelude to constant hijinks; as we saw with _Ranma 1/2_, a water-change mechanism can be exploited in countless ways. SnH uses it a few times and forgets it. Or, the first time San does her 'chivalry is spelled the same way as mermaid' shtick, you might infer that this will be ripe for comedy as irrelevant coincidences of kanji lead a headstrong San into considerable silliness. Nope. The yakuza/Mawari angle is weirdly downplayed.
And SnH has entire misfiring episodes: episodes 12 & 13 would've been better combined; episode 14 seems a bit thin as a standalone episode (an entire episode on mermaid fear of cats?) and like it would have worked much better as a subplot in another episode; episode 15 is a complete waste, as is episode 21 (both focused on the president character, who should've been mercilessly cut); episode 20 is just uncomfortable and not nearly as funny as the creators apparently think; episode 23 tries to resolve the Masa character but just leaves him in a weird limbo state vis-a-vis his sister which makes one wonder why ep23 even existed when Masa was fine on his own as a 'cool gangster'/mentor character. 26 episodes is a lot of time to work with, but if you want a good comedy, you can't leave so much slack & worthless material. (A similar problem sabotaged _Nichijou_: too much space to fill, not enough top material.)
So you couldn't really compare it to better comedies like _FMP: Fumoffu_ or _Azumanga Daioh_ or better dramas like _Toradora_.
