The other boys that the protagonist sees at the beach could symbolize a kind of peer pressure or a standard that he feels he needs to meet. Their presence and their ability to swim through the tunnel before him drives him to take on the dangerous challenge.
The tunnel itself is a major symbol. It represents the boy's rite of passage, his journey from childhood to a more grown - up state. It's a physical and mental challenge he has to overcome.
The beach can be seen as a symbol of the boy's comfort zone. It's where he starts from, and leaving it to swim through the tunnel is like leaving his safe and familiar world behind. Also, the ocean is a symbol of the vastness and unpredictability of life that he has to face during his growth.