Why does Brian want to go out to the plane?

Brian wants to get to the plane and see if he can retrieve the survival pack.

The tail of the plane sticking out of the water. There it was, the tail sticking up. And inside the plane, near the tail somewhere, was the survival pack. It must have survived the crash because the plane's main body was still intact. That was the picture—the tail sticking up and the survival pack inside—right there in his mind as he dozed. His eyes snapped opened. If I could get at the pack, he thought. Oh, if I could get at the pack. It probably had food and knives and matches. It might have a sleeping bag. It might have fishing gear. Oh, it must have so many wonderful things—if I could get at the pack and just get some of those things. I would be rich. So rich if I could get at the pack.