Why did Brian swim out to the tail of the plane?
After the huge storm and the tornado that rips through his campsite and destroys everything he worked so hard to build, Brian is momentarily taken aback. But he quickly realizes that he has to get back to work and that he has to start re-building. As he begins the process, he sees the tail of the plane now sticking out of the lake.
As he thinks about it, he remembers the emergency supplies bag that was behind him in the plane and knows that he might be able to get it. Brian is very excited about the things that might be in it but he has to overcome a number of obstacles in order to get the bag.
First he has to be able to have some sort of stable platform to work from while out in the middle of the lake. He eventually figures out how to make a raft that solves that problem.
Then he finds out that he can cut through the skin of the plane with his hatchet so he knows he can get in to get the bag.