Brian wants to find a lake in front of him because he figures a lake is the best place for him to crash the plane he is now flying.
In chapter 1 of the book, the pilot who is flying Brian has a heart attack. Brian immediately takes control of the plane, but Brian is not a pilot. He doesn't know where he is or how to read the instrument panel. He's afraid to try and turn the aircraft around because he believes he will lose control of the plane. Brian's survival "plan," then, is to keep flying straight until the plane runs out of fuel. When that happens, Brian will try to crash land the aircraft. Brian needs a lake because he knows trying to land in the forest's trees will tear the plane apart. Brian believes a lake offers a better chance at survival.
Just the lakes, and it came to him that he would have to use a lake for landing. If he went down in the trees he was certain to die. The trees would tear the plane to pieces as it went into them.
Brian needs a lake in front of him because he isn't capable of turning the plane and maintaining control.