This movie is filled with beautiful animated depictions of the sea and the many creatures who inhabit it. Unfortunately, the beauty of the animation doesn't salvage what is a slow-paced and overlong movie that tries too hard to condense the headier themes of the manga into a feature-length film. Also, these "headier" themes -- the universe needs balance, everything in the universe is interconnected, we're all made of "star stuff," etc. -- isn't exactly breaking new ground, as many of these ideas can be found adorning the throw pillow section of the nearest Bed, Bath & Beyond. In terms of the teen melodrama aspects to the movie, the "fish out of water" saying is a literal depiction here, and while sometimes anime makes creative use of applying these terms to the magical realism of the worlds they depict, in Children of the Sea, it begins to grow increasingly tiresome.
It's the classic example of an anime that tries to do way too much, and the sheer volume of messages and side stories and backstories work against each other. The sheer volume of what's being communicated, as it's paired with beautifully hypnotic images of, say, tropical fish, creates a kind of numbing effect. The coming-of-age story gets lost in the morass of Big Ideas and Grand Concepts. And it's not to say that these thoughts on life, the universe, and the deep blue sea shouldn't be expressed, as stale as some of these ideas have become for overuse, but the lush animation would have been better served by using this movie to address some of these messages, and not all of them.