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Right, this is a "coming of age story" i.e., it's about the enchanted quality of puberty. I'm a sucker for these stories, and I really enjoyed this book. I'm also a fan of the borderlands motif - the seams where worlds meet - and a major feature of this story is loving descriptions of tidal marine life.
I think it goes well with "The curious incident of the dog in the night-time"; they're both stories of awkward boys amazed by the world, who have intellectual interests.
Oh there's some sort of messianic sub-plot. Or, maybe it was the main plot device... I can't really remember. But it's actually pretty unimportant to the book, so don't let it scare you off.
How can the main plot-device be a trivial element in a book? Well, maybe it's supposed to be the allegorical skeleton which shapes the entire book. I don't know. What I do know is that at a page-by-page, paragraph-by-paragraph level, descriptions of marine life, the physical setting, and the emotional development of the boy form the primary content of the book. Those are what it's worth reading for.