Why does Jerry continue to try to swim through the tunnel and not swim back?

Once Jerry is far into the tunnel, there really is no choice but to continue pressing forward. The tunnel is fairly narrow, so there is not enough space to move his arms in the wider way he would need to in order to move backward. Moreover, the tunnel is dark and he cannot see for most of the early part of his swim, so he would not be able to tell where he was going, especially moving backward (and against intuition). Swimming backward would take so much longer that he would waste precious air. Further, completing this task is a major point of pride for Jerry. He doesn't want to back out and give up—he wants to make it all the way through to prove something to himself: that he is as capable, independent, and brave as the older boys who did it before him.