Lucas' body has been washed onto the ocean and found the next morning when caught up in a fishing boat's propellers, but the gunshot wounds in his head have dried blood on them that hasn't washed away.
Presumably Camille, as a native (at least as presented) on beach and water matters, has Poole crowded into the bow of the rowboat while she, evidently, is seated extremely close to him, and definitely on the same end with regard to both gravity and buoyancy. Poole has every reason to be apprehensive, at the least.