I was a lifeguard and swim instructor assistant in high school (and college). I had to pass a test to become lifeguard-certified and take CPR and first aid but did that the spring I was 15 so I could be employed that summer. I also was a swimming lesson instructor assistant (minimal testing) that allowed me to assist those who were certified instructors. I later became a certified instructor.
But I was able to find summer employment first as a substitute lifeguard (worked 4 days a week usually just as a sub) and then later as I was hired on for full-time summer employment, i would work 5 to 6 days a week (I would work 4 full days and the offer to sub a couple of afternoons or evenings to make extra cash).
If a person is a good swimmer and there are pools nearby, this is a great summer job option. Once I became a certified swim instructor, i could also offer private swimming lessons and make even more money.