Jerry simply has to swim long and hard enough underwater to make it to the other side before coming up for air. As the tunnel has no air holes along the way, this is a "win all/lose" all situation.
Swimming through the underwater tunnel is a challenge Jerry is taking on in secret. He has nothing to prove to anybody except himself.
Risk taking is not really so rare during puberty and adolescence, though, and some psychologists consider it a natural and even necessary part of the maturing process. The problem is that danger is very real and uncompromising: some kids push themselves too far and die or seriously injure themselves by taking on a foolish "dare."