Hey there Valerie,
I have taught aquatic group class, rehab and pt for about 16 years. While going through land PT for ACL hamstring graft designed and did my own deep water sprinting/HIIT program for 6mos til I could do the same on land.
Firstly: to lend credibility and better education to represent yourself in this area of specialization I would highly suggest an aquatic certification. Properties of the water are completely different than land based training. There is hydrostatic pressure, buoyancy,HR doesn’t translate the same, ideal pool temps for different diseases and ortho problems to consider. And that’s just scratching the surface of the many differences. Or, if you really enjoyed PT, get a PTA degree and look for employment with a PT organization/office whose focus is aqua. The opportunities are endless.
Once there are credentials in place, it would be easy to market yourself to private individuals in their homes, local condos, residential communities who have indoor pools. Every Y and JCC around the country also have pools. Most don’t “rent” them.
Here are a few links that may help. Best of Luck!!