In "through the tunnel," the underwater tunnel that jerry is determined to swim through may be seen as a symbol for

What I think it is, is that the first, safe and happy beach is most likely him living his best life with his mother, and all goes well.
The next one represents what becomes of his life. He doesn't have his mother, and he's becoming an adult.
The tunnel is perhaps what Jerry goes through to gain access to this new unpredictable future in store for him.
Hope this helps!