Much as it pains me to say this (I loved the first two books in the series): I don't think this book was necessary. I don't want to say 'boring', because that's so harsh and also not quite true, so maybe I'll call it 'not engaging enough'. 'Too full of stuff the story doesn't need', maybe. Or, 'too many twists and turns'? Look, what happens is: We're on the ship, we're going some place and then we 1) have a problem, 2) get out of it, 3) have a problem, 4) get out of it, 5) have a problem, 6), er... get out of it, 7) have a problem, 8) oh look, book's over.
I want to know how the story ends, still do, but it could've done so already. It SHOULD have done so already.