Eighteen Years, to the Sea (1979)

Kei and Atsuo were both enrolled in summer school. Kei was an honor student, and Atsuo was the polar opposite - but opposites attract. After school, the two head to the beach, where they find another student from their school competing against a local gang member, to see who is able to hold his breath longer under water. Overwhelmed by the excitement, Kei and Atsuo decide to try themselves.