I worked at SwimKids 8 years, and left only because our family business needs extra hands.
Pros
The water is warm. The instruction method is step-by-step, progressive, and supportive of a child's emotions. The methods will be familiar to a swimmer, and the learning curve less steep. I was not a swimmer when I hired on. It took me a while to get what I was doing. The more effort you make in learning the curriculum, the more intuitive understanding you'll have of your students' needs. Working with children is rewarding. The parents were supportive and patient. The parent other instructors were enjoyable, high-school and college age, some 30s and 40s, and some my age--all had something to offer. The office staff is helpful and friendly. Over the years, the concept of deck support became the rule, so there is usually a person poolside to help with student/instructor/parent questions that come up. This change was very nice. The hours are flexible seasonally so students can adjust work to fit their class schedule.
Cons
Depending on your stamina, a 5 or 6 hour shift may leave you tired for the next activity in your life. 3 or 4 hours is more do-able. Be proactive in the care of your hair, skin, nails and swimwear, or the wear and tear on your body and swimsuit will make the job less enjoyable. Find easy care methods and do them before and after every shift. If you don't like the job in spring, summer and fall, you won't like it in winter when it is cold. This job is fast-paced and highly scheduled; you must run your lessons on time. There is rarely time for chatting--you'll get to know your co-workers as you swim past them, or after your shift. If you're looking for friends, it takes time. If you're looking for Mr. Right, the male instructors I knew were great guys, but they were few. The employee turnover rate is largely situational due to the vigor required for the job. Younger people have the needed energy, but young people eventually go off to college, finish college and get a job, or get married.