Hello Karen Maestri,
The benefits are plentiful. Besides helping arthritis and post rehab, there is also cardio, strength, flexibility, buoyancy help with balance moves, just about everything except maximum bone building because of the buoyancy which is like a spotter for the swimmers.
Maybe the group session camaraderie is the most important benefit.
Natalie aka NAPS 2 B Fit.
IDEA has a lot of good information on this topic for you:
Hope this helps.