Examine your current interests and skills.
If you enjoy physical fitness and sports, that is certainly a potential career path. If you feel you aren't good enough to be a professional athlete that still leaves:
Physical therapist (medical injury/surgery recovery)
Physical trainer (ie working for a sports team)
Personal Physical Trainer (individual coaching, either on your own or something like a fitness center)
There are also hospitality careers. Lifeguard on a Cruise Ship for example.
And there are even more scientific/medical routes. Orthopedics, Prosthesis, Human Mechanics/Ergonomics, etc.
Don't have to limit yourself to water/swimming either.
If you are thinking of something else, what are your interests?
You can also look at any current job you have as a way to learn interpersonal and professional behavior. I worked at swimming pool when I was 15 now that I think about it, administration and concessions (front desk/food)