Water therapy and exercising has been proven to be beneficial to people who suffer from Parkinson’s disease. Exercising in a water environment helps with issues such as balance coordination and joint mobility and/or pain. Since his doctor suggested it I would say that he has the clearance to do so. I would also seek the advise of a Physical Therapist as well in order to get a more detailed information and suggestions on exercises, duration and intensity. The same applies for you since you have knee problems. I hope this helps.