“Student” would be an appropriate description for your occupation. You could also use “Lifeguard” since that was what you did for work. Either is fine. The description entered as your occupation will not change your tax return in any way.
Is student considered a job? Students are treated the same as other persons; that is, they are classified as employed or unemployed if they meet the criteria, whether they are in school on a full- or part-time basis.