Where did Brian try to aim the plane for his best chance of landing in Hatchet?

After the pilot of his small plane has a heart attack, Brian focuses his efforts on landing a plane in a lake. He feels that this offers his best chance for surviving impact, and takes over the controls in a desperate attempt to save his own life.

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.

When the reader joins Brian's story, they are quickly thrown into a scenario of peril. Because the situation is brought on so early in the story, this is one of the first instances where the reader really gets to know who Brian is. By learning of his desire to land the plane on water, they also learn that he is smart and has at least a vague understanding of what it will take to live through the ordeal.
Having a main character that exhibits intelligence can be crucial to the story. Keeping the reader interested in seeing what happens and how the character is going to work themselves out of the situations in which they find themselves helps to keep the story suspenseful and interesting. When Brian decides to land the plane on the lake, the reader has hope that his journey through the rest of the novel will be successful, though difficult.

The answer to this question can be found in chapters 2 and 3. The book doesn't waste any time with putting Brian into a harrowing situation. The pilot of the little bush plane has a heart attack at the end of chapter one, and Brian is forced to fly the plane and consider the various scenarios in which his plane is no longer flying. Brian knows that the plane can't fly forever. The fuel load limits that; therefore, Brian knows that he is going to have to land the plane somewhere. In his mind, putting the plane down on a lake is his best chance for survival. He knows that a lake is his best chance for a clearing free of trees that would otherwise cut the plane to pieces.

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.

Chapter 3 has Brian put the plane down onto/into the water. His lake plan works, but it only just barely works.

So far to the surface and his lungs could not do this thing, could not hold and were through, and he sucked water, took a great pull of water that would—finally—win, finally take him, and his head broke into light and he vomited and swam, pulling without knowing what he was, what he was doing.