What a diverting, vivid, beautifully detailed story. The leg serpent is an inspired creation - she may have been borrowed, but you breathe life into her. The setting, the aftermath of the cyclone, the sea, the seasickness, Severus's ship, all are described in brisk and brilliant language that summons everything to mind. I love the sensual clarity. I love your wry, self-deprecating, but competent Harry who "imprints" almost instantly on Snape. The scene of his discovery of Snape trapped in the undersea grotto - the moment when the arm floats out and Harry spots the Dark Mark - is quite eerie. The image of the two of them floating naked for hours in the water, Snape utterly helpless and Harry burning with protective determination, has that lovely quality of h/c eroticism that prepares us for their abrupt and intense response to each other, once they're safe.
And these lines made me grin: Harry couldn't even pretend to himself that this had anything to do with his saving people thing. This was a kissing Snape thing.
Neville's plight was rather horrifying, and at first I feared it would overshadow the fic. The single tooth caught in the deck of the ship! But you managed to use a light enough touch, and Harry's scrappy heroics and resourcefulness lured me onward, as well as being in themselves a pleasure to read (considering how maddeningly dense he could be in canon). I thoroughly enjoyed your OCs, including the seasnake, and there were innumerable small bits of local color that made the fic teem with life and intelligence. You clearly put some thought into the various kinds of magic; your descriptions of the gillyweed and of Snape's toxin-saturated body are rather striking, for instance. I've now got the image stuck in my head of Snape with dreadlocks. I just wish there could have been a bit more room for Snape and Harry together. You do a wonderful job of catching the banter between equals here, but there's also that streak of vulnerability in Snape that's hard to pull off convincingly and is irresistible when done well. I wanted to see more.
In short, I enjoyed this fic so much I had to come back and read it again. :)