Bad knees

I have bad knees (6 knee surgeries in the last 14 years, 3 on each knee, I have a rare form of arthritis and zero cartilage in both knees).  My Rhueumatologist says to do eliptical or stationary bike since they are very low impact on the knees, but she would prefer I did water walking or water aerobics.  Walking on the treadmill is ok in her opinion but not too fast, or at too much of an incline.

My orthopedic surgeon also concurs with the water aerobics, and also recomends any type of strength training you can do to strengthen the quads, hamstrings and Hip abducter muscles.  Thoses muscle help support the knees and absorb some of the shock of each step, like the cartilage used to do.

You may want to consult with your doctors to find what specific exersices they like. 
Good luck with whatever you decide to do.