Hello and thank you for your question. It's a resonably "large" question as for scope, but I will do my best. The overall potential learn to swim market is pretty huge. It is every current and future person who does not yet know how to swim. EVERYONE should know how to swim. It is the ONE sport skill that will save a person's life; and then it leads to so many other life-enhancing and life-enjoying opportunities and pursuits, whether it be for sport or for leisure and recreation.
You are correct in your listing of the types of programs that are currently offered. I believe the current trend is still pretty organic and local which seems to be leading to decent to moderate growth in the overall market. Certain communities have been more successful than others and is very dependent on program leadership.
The market is very labor-intensive - for every few new clients in a class, you need another teacher. There seems to be very little passive income. You're only earning revenue when facing clients. Quality training is a must.
I hope this helps you get started on your discovery. Good luck.