Qualifications
You'll need Red Cross certification in CPR, first aid, and lifesaving.
You'll basically have to pass the lifeguard test for the place you'll be teaching at; in community pools and camps, it's often the lifeguards who double as swim instructors.
You not only need to swim really well and understand all of the strokes, such as freestyle, breast stroke, back stroke, butterfly—but you'll also need to be able to communicate and visually demonstrate how to do these to others.
You'll need to be able to explain the mechanics of rotary breathing to small children along with the concept of floating and buoyancy.
You'll have to be really, really, really patient. It takes some people a long time to grasp the concept of swimming.