20-Year Prospect
In twenty years we will still need to teach people how to swim...provided global warming hasn't completely dried up our water supply.
Odds are, that won't be the case in twenty years (it might be in forty, but we certainly hope not).
You'll still have all the same opportunities to teach swimming as you do now. The going rate may go up a bit with the cost of living but don't expect any dramatic change. On the plus side, this is something you can do even into an advanced age, if you wanted to that is. Being in the water is easy on those joints.