It was pretty good, though it did have some flaws and annoyances. I'd normally rate this like a 7.5 or an 8, but it gets a boosted rating for a couple of features, namely that it doesn't have any NTR or annoying worthless crap like that that plagues the genre these days, and that it actually has a true harem ending.
That being said, it was not without flaws. The MC turned into a dunce whenever it came to his college-friend (the only one he knows before he moves to the said Mermaid Town). He has absolutely completely friendzoned her to a ridiculous degree for... no reason whatsoever? He also develops sudden memory issues when it comes to critical thinking.
Example: a prank gift from one of the other female leads (SM toys) gets put into his bag. She finds them, thinks he's into SM, promptly gets embarrassed. He goes home, sees it, and instantly puts together why she was acting embarrassed. Then literally days later she invites him out, proudly proclaims she's into SM, drags him into a adult toys store, and starts buying stuff. She then loudly proclaims (on the street) that she's a submissive female and she needs him to be her master. He says no, because he doesn't think they can be friends after, and she says THAT'S TOTALLY FINE! And drags him to a hotel. He... somehow... hasn't put any of the pieces together.
Speaking of the S&M, there's a ... lets call it a rather lengthy subplot with a ton of scenes. Most are with the MC being dominating (which is more pleasing to some of the women than others), but there's also some of the reverse, with him being dommed himself (pretty hardcore too), so if you don't like that you might find yourself skipping chapters (or dropping it entirely). Fortunately it's only temporary, just like with the co-worker when the truth eventually comes out.
Also, said college friend is really aggressive when competing with the absolute darling of the story, the 'main love interest', so she comes of as incredibly aggravating. To the point where you might start to hate her, as she seems to be leaning into the trope of 'annoying-a$$ CHF who keeps getting in the way of Best Girl' which is all too common.
As for the harem, it's a weird one because they literally do not know about all of each-other until the LITERAL LAST CHAPTER. Different pairs know about each other, but not a single member knows about all of them. There's the MILF house-owner, who in some ways is the least complicated, but she has her own personal story going on the most out of them. Then her sister, a kindergartener teacher (glasses MILF), who has the least emotional connection, but probably the most varied kinks and scenes. The college friend fellow intern is mentioned above.
Lastly though, is the only reason this story isn't absolute trash. She's a darling. Adorable, sweet, strong and friendly when she needs to be, and not phased by the 'mean girl's' overreactions. Honestly in a non-harem ending, she'd be the winner for sure.
Speaking of the ending, I'm fairly sure this was almost axed. The end was definitely coming - plot points were being wrapped up and the story was heading towards the end, but it was still done SUPER abruptly. As mentioned, each of the girls was in the dark until the very final chapter about at least one other girl, and really at no point was sharing ever mentioned (there was talk about playing fair and waiting for him to choose one and such, and a few threesomes, but those were definitely temporary).
It feels like it was supposed to be single ending (which would've sucked, but at least 100% the best girl would've won) and the author just pulled a true harem ending out of his a$$. Rushed, and abrupt, but hey, at least it was a happy ending.
TL;DR: So yeah, story with flaws, but still gets a higher rating for being no-NTR and a harem end.
I'm so sick and tired of stories these days having great harems and then absolutely flubbing the ending, that I'll 100% take this with a mediocre-written ending, but still being harem. Would rather have a poorly written harem end than a poorly justified single ending with all the 'losers' just randomly being written off lol.