Qualifications
You have to take a rigorous test that's...how do we say this? Hard. It's hard.
If you're working at a pool, you'll have to pass the Red Cross Water Safety test. The tests aren't easy, and typically run you through an intense series of swimming and water treading drills.
You'll also need your CPR and first aid certifications, and most places will require you to take lifeguard training classes, too. You're a step ahead of the game if you swam or played water polo in high school. Did you swim in college? Great. Then you should be a shoe-in for the job.
For a beach lifeguard, the test is akin to entering an iron man contest and finishing in the top two percent. You'll be expected to do things like swim 100 yards in a pool under a minute and race 1,000 meters in the icy, tumultuous sea. There's also a foot race and chest carry until there's only 60-80 men and women standing out of the hundreds that apply. Hope you're up for it.
You need to be fast on land and in the sea (or pool), have a keen eye for detail, and a good work ethic. You also better be brave and have strong communication skills (even if you're just yelling, "Walk! Don't run!" all day).