Shark Beneath the Reef by Jean Craighead George

Like his father and grandfather before him, 14-year-old Tomás Torres dreams of catching a great shark in the Sea of Cortez -- and he will catch it, although there are other things he should be thinking about. With an education, her could someday become a marine biologist. Tomás's family want him to stay in school. But Tomás knows he will be more help to them if he leaves school now to become a fisherman. Should he drop out?

The choice is Tomás alone -- a difficult one for a boy just becoming a man. It is only underwater, in a confrontation with the fisherman's greatest prize and worst enemy, that Tomás finds the strength to make his decision.