Why, exactly, did they have to destroy the island to get away from the "dinosaur." If the thing could've chased them across the lake, it would have. I suppose it made for an "exciting" finale, but there's no logical reason for it. Unless you consider "Lizard must die!" logical.
The "we have to make camp" scene is odd. After the perilous (ha) snake attack, Ralph says they should stop until daybreak, I assume due to concerns about nocturnal predators. Fine. But why bed down in the area they must know is crawling with killer alien snakes? And finally, despite the original reason for them having stopped (i.e. Ralph's apparent concern about attacks), all four team members participate in the shelter-building with nobody posted on guard duty, meaning everyone could've been attacked and killed by something sneaking up on them at any time while they were focused on work. They do take turns watching, but only after the shelter is constructed.