Well, Brian wanted to get into the lake to salvage the items inside the airplane. (Survival Pack) You see, by building a raft, it would help Brian to get closer to the airplane-which is currently located in the depths of the lake. By building the raft, Brian would have a chance to...
A.) Get the emergency transmitter (a.k.a. the device used at the beginning)
B.) Break into the emergency food supply
C.) Retrieve some items from the emergency supply (Ex: tools, lighter, ex.)