Salary
Average Salary: $22,000
Expected Lifetime Earnings: $918,456
The money is steady but not very good. There are pockets of wealthy neighborhoods where an instructor might make a livable wage, but most of the time you'll have to get accustomed to $12 or $13 an hour (source).
However, if you can get a job with a posh country club as a lifeguard/swimming instructor or with a private swimming school, you can probably make a bit more, closer to $16 or $17 an hour (source).
If you work at a summer camp, there's also end of the summer tips to look forward to from grateful parents, as well as future gigs as a private teacher in backyard swimming pools. That sort of work could run as high as $20 to $25 an hour, depending on the neighborhood.
Overall, you're looking at a yearly salary of between $18,000 and $38,000, not exactly enough to retire on (source).
However, get a job coaching a swim team at a high school, along with maybe teaching a class or two, and you're looking at a teacher's salary in addition to $6,000 or more to coach the team. Plus you get a teacher's pension. You're looking at around $62,000 to start (source). That's certainly livable.
A college swim coach would only make around $40,000 on average, which is not much better than teaching at the Y or at a camp.
There are other kinds of swim coaching you can do if you're willing to get some education and certification for aquatic therapy. There are only a handful of schools that specialize in this, though some insurance companies will pay for an accredited aquatic therapist.
Salary for an aquatic therapist is typically $63,000 (source).
Many aquatic therapists also teach swimming to make even more money. Keep your options open, that money's got to come from somewhere.