This is basically the first book, except with boats, treasure, and cannibals. Hiccup is terrible at being a Viking, a problem comes up, nobody listens to Hiccup, things go wrong, Hiccup uses his smarts to save the day. Oh, we find out that Hiccup's left-handed, though. So there's that, I suppose.
Otherwise, nothing really changed.
Hiccup saved everyone from the giant sea dragons in the first book, and literally after one sentence, everybody forgets about Hiccup exploding a dragon as big as a mountain, and they go back to laughing at him. It's like the only one who can even remember the sea dragon incident afterwards is Fishlegs, and nobody cares about Fishlegs.
On the plus side, this is one of those series where you don't really have to read every single book to understand what's going on. Enough back story is given to sum up the first book, and explain what's going on.