Not a senior here, yet… (though I do qualify for AARP membership!) but I do have knee problems and am overweight (which doesn’t help). I’m finding that an elliptical trainer is a lower-impact form of cardio exercise, without all the hassle of finding a pool, having to change into/out of swimsuit, etc. that you have with water aerobics.
There are a number of different styles of “ellipticals” however, and some of them really feel wrong on the knees, so it’s important to find one that works.
Of course your mother might not “enjoy” it - having music to listen to, or a TV nearby to watch, can help a lot.