This is your archetypal 2000's harem manga, 'nuff said.

...The first half is quite good for this genre and deserves at least a 7/10. Unfortunately it progresses in exactly the direction you won't want it to (i.e. nothing happens) and the resolution comes out of nowhere and feels like a step backwards. But the girls are cute, it's kind of funny, it's an easy read, and it (usually) does a good job of just taking itself seriously enough to feel like a real story without getting bogged down in drama.

Let's be honest, you're reading this for the ecchi. Umi no Misaki delivers on that front for sure, by gradually building up the sexual tension and going a bit further in each encounter, throwing in some extra girls here and there, and all around it's a good time. But for some reason, around the 2/3's mark when we're finally getting to see the things we've all been waiting for, it just stops with the ecchi almost entirely and shoehorns in some crappy drama that consumes the rest of the series. It's very disappointing because at that point, the story feels like it's been laying the foundation to a proper ecchi series, where the characters are all in the right setting and states of mind to get some juicy romance. But then it just... doesn't happen. There's a BIG DRAMA that gets quickly resolved and the story ends.

Story-wise, it kinda sucks but it's a quick read, so it's not too bad? Seriously, the dialogue is VERY light and simple, I've never read a ~130 chapter manga so quickly in my life. So even though the plot is paper-thin, it progresses quickly enough that it stays fresh. Honestly didn't find myself bored until the end, mostly because I could get through 10 chapters in as many minutes. The characters are fun and have their own personalities, although Soyogi and Shizuku are basically the same person, and Karin (the most interesting character by far) gets the short end of the stick most of the time. Nagi is about as bland as they come and spends most his time passively observing the girls as they lose their minds with jealousy and try to take his pants off.

Our main dude, Nagi, is the "remarkably level-headed"-type, which is the most tolerable of harem protagonist archetypes. He basically just earns the affection of every girl he comes across, simply by being less insane than they are and also not an asshole. He has no interests, backstory, motivations, or personality. But he's kind of a nice guy and that's enough for three women to spend their lives fighting over him. Whenever one (or all) of the girls start losing their minds with jealousy, he pats them on the head and tells them to believe in themselves, and they transform into a little puppy and go back to fighting over him.

Do I recommend this? If you're already reading reviews for this manga, then you're clearly already the type to enjoy it. So go ahead and read it until you get bored, then stop. There is no satisfying resolution, nothing exciting is revealed towards the end, so you're absolutely free to give up whenever you lose interest. It's fun if you want some summery, nostalgic ecchi, but it's probably not enough to actually crank one out to and you're not gonna find anything particularly memorable. Try it out, it's fun.