The story begins with Nagasumi, a seemingly average Japanese high schooler, drowning in the ocean. He's saved by a mermaid, Sun, but unfortunately for both of them the merfolk have very strong laws against being seen by humans. Complicating the matter is that Sun is the only daughter of the leader of a Yakuza group, meaning these laws can't just be overlooked. And what is the punishment for breaking these laws? Death, for either the human or the mermaid. Luckily there's a 3rd option that you can guess from the anime's title; marriage. If Sun and Nagasumi are married, they'll be family and the laws won't apply. And so begins their bumpy road to a relationship, as Sun's overprotective father and his Yakuza attempt at every turn to kill Nagasumi, Sun moves in with him, they struggle to adjust to both of their respective worlds and Nagasumi desperately tries to keep Sun's secret a secret from others like his odd "friend" Chimp or his police officer aspiring childhood friend Mawari.
It's a unique set up for a rather typical romcom storyline. Mermaids don't get much exposure in any kind of media these days so it feels fresh despite using a creature we're all familiar with at this point and they've got some interesting takes on mermaid culture in modern society.
Ultimately however it's never the setting that makes a romcom, it's the characters. And this show does a great job with them. Nagasumi is a fairly typical lead, a decent guy that you occasionally feel sorry for considering the stress he has to go through because of his new situation. But he's not perfect; there are times when he's a dick, or lazy, or inconsiderate. Basically he's human and, sadly, that's a breath of fresh air for this genre. If you're tired of seeing the typical white knight that stumbles into "perverted" scenarios every episode, then you'll enjoy Nagasumi.
Sun, as his opposite, is the true star. As I said above, she's a ditz, maybe even stupid at times, but she's got a heart of gold and legitimately cares about Nagasumi. It's tough not to like her. And...that's really all I need to say about her. She was my favourite character and considering she's a lead, that's great.
There's a handful of other characters, all fleshed out and feeling like actual people even if some are a little...eccentric. Sun's dad is a bonafide psychopath, but at his heart he just loves his only daughter. Chimp is, well, Chimp and Lunar, despite seeming to be mentally unstable, really just wants to be loved. There's more, but I'll leave it here since most of the fun is in discovering and experiencing these characters for yourself.
It's at this point too that I want to say I feel this is a harem anime done right. That might be an odd thing to say, since all a harem would need to be "right" is a guy and a bunch of girls obsessed with him, but its always too unrealistic for me to suspend my disbelief. The girls are shells, filling one trope or another and existing purely to populate the harem. This isn't the case in My Bride Is A Mermaid. All of the girls, despite having and/or developing feelings for Nagasumi, are characters in their own right. They're not defined by their feelings for Nagasumi and it's an important distinction. You can imagine them existing without him, which isn't the case in other harems like, say, Rosario to Vampire. I hate to keep using this term but it's just a breath of fresh air.
The animation is good, if nothing special. It's typical of what you expect of a modern anime without standing out. The music fits, keeping the tone upbeat and generally working to keep in line with the light hearted comedic half of the show. The VAs are at the top of their game as well, with Lunar's VA sounding like she had a bunch of fun with the role.
Overall, this is a must see for romcom fans and I feel you should give it a try even if you're not into the genre. There's few, if any, better examples that you could find.
