At its core, "Through the Tunnel" is a coming of age story. Jerry and his mom are on vacation together, and like most teenage boys Jerry wants some freedom and independence from his mom. At least that's how I felt as a teenage boy. Jerry loves his mom just as much as he ever did, but he feels that his mom's presence makes him appear less independent. It even makes him feel less independent. He sees the other boys going about their fun without any mothers around and Jerry wants some of that. He wants to prove to himself and to the other boys that he is no longer a boy. He sees himself as a young man, and he wants the other boys to see him that way too.
They looked down gravely, frowning. He knew the frown. At moments of failure, when he clowned to claim his mother’s attention, it was with just this grave, embarrassed inspection that she rewarded him. Through his hot shame, feeling the pleading grin on his face like a scar that he could never remove, he looked up at the group of big brown boys on the rock
and shouted. . .
You can see from the quote that Jerry is sick of the patronizing "little kid" look that he normally gets. That's why he trains so hard to make the swim. The tunnel is a metaphorical gateway to becoming a member of the young man group that the other boys belong to.