With the movie adaptation of this coming out in the next couple of months, I might as well insert my five yen on the manga. Which I read almost a year ago. ...Fortuantely, I know what I thought of it then, and I know what I think of it now! And hoo boy, what an amazing manga this is!
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Story- ACTUALLY Deep/10

There are a lot of works of fiction that people consider deep that I... don't. To a lot of people, human life is unequivocally fascinating and complex. Personally, I find human life to be overrated, and this manga's speaking my language! What starts out as a supernatural mystery manga is actually a profound love letter to the beauty and majesty of the Earth and its oceans. And when I say profound, I mean profound.

To be fair, the esoteric glarble I mentioned just now is merely my interpretation of the story. But the way this is crafted makes me want to believe that it was the author's utmost intention and why I'm willing to give the accuracy of the manga's marine biology factoids the benefit of the doubt.

This manga gives us a simple wake-up call: We ain't special. There are a lot of really cynical works of fiction that do tell that point, but those almost come off as even more melodramatic and pretentious than a teen romance. Kaijuu no Kodomo gets the message across plain and simple. We are part of a grand and beautiful tapestry of life, and most of us have been ignoring it becuase of our own superfluous issues. Heck, we know more about the vacuum of space than we do about our oceans!

However, this story has a lot of disbelief to suspend. Stuff just 'happens' because 'reasons', and we have to deal with it (to put it in a spoiler-free manner). I personally interpret it as a m-m-metaphor (that word is so poisonous to me these days) for how we, humankind, are too small-minded to comprehend the massive beauty of nature.
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Characters- 7/10

The characters aren't as important as the main story. Like us, a lot of these people are just along for the ride. The most interesting characters are the titular children, Umi and Sora, without a doubt. They're the ones who drive mainly drive the narrative along, while the main character, Ruka, just passively follows them. The tattoo guy is a bizarre take on a main 'antagonist' who instead comes off as... just a guy.

Also, Ruka's mom has it going on. Just sayin' (sorry).
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Art- 9/10

This manga is beautiful in a weird way. There are a lot of times where the characters aren't anatomically correct, but it makes up for it with gorgeous landscape shots and detailed textures. This is a visual manga, having a lot of chapters with almost no dialogue. Who needs it? Let nature tell the story sometimes.

Some people will probably find the movie more visually pleasing out of personal preference, but nothing's beating the occasional color pages of the manga, which have a (no pun intended) WATERcolor motif to their shading and textures.
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Overall- 9/10

This is one of the most beautiful manga I have ever read. Its message is simple and powerful while also not pretentious. I would have rated it a 10/10 if it weren't for the semi-loose ending.

But as much praise as I have given this, I can't safely recommend it to you. It all depends on the kind of person you are. I know a lot of people will find this to be utter nonesense, sadly. There isn't even a real conflict in the narrative!

The only basis through which I can recommend Kaijuu no Kodomo is- oddly enough- the Rivers of Light at Walt Disney World's Animal Kingdom (plus, the tattoo guy reminds me of Joe Rhoades). That performance tries to touch the soul with its homage to land-based life similarly to how this manga does with marine life. If you love that show, then this manga is 100% up your ally! But if you at least have an open mind going into it, then it should be a profound experience.