The shark can push the player under the raft where they get stuck and drown. When it happened to me I was in the water with my back against the raft where the shark swims up and attacks me. I stab it 3 times with the spear and it swims forward while diving- pushing me under the raft. I can't break the foundation above me because the bobbing motion of the water throws me off-target before I can finish breaking it.