The constant rain had turned the soccer field into a mire, filled with slushy mud that would trap your feet and pull you down.
Neither coach wanted to let her team play in such a swamp.
I believe these are the two sentences that show you what more means. Hope I helped!
The usual interpretation is that the "safe beach" represents the comfort and security of Jerry's childhood and his mother and the rocky, "wild beach" represents the unknown adolescent future without his mother. The "tunnel" is a challenge or rite of passage or test Jerry must pass to gain access to that future.
