What mistake did Brian make in the use of his hatchet?

After the tornado, Brian attempts to get his shelter repaired. As he's looking down the shoreline for wood, he notices the tail of the plane sticking out of the lake. It occurs to Brian that if the plane is still intact, he may be able to retrieve the survival pack. Brian constructs a raft and swims to the tail of the plane, where he uses the hatchet to cut into the aluminum. As Brian attempts to bend a piece of the aluminum, he drops the hatchet in the water. He immediately recognizes his mistake and recalls how important the hatchet has been to his survival. Luckily for Brian, he is able to dive deep into the lake to retrieve the hatchet. This time, as he uses the hatchet to cut the aluminum, he is much more careful with his prized possession.