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.
I have been teaching water aerobics for many years, and many participants have benefited greatly from the aquatic environment.
There is a difference between water walking and water aerobics, though. The latter is usually in a class format, often to music, and while often very enjoyable, may be too much for an entry level. In water walking, you would typically do different walking patterns (backwards, forward, sideways and such). It is usually self-paced and outside of classes. Some physical therapists specialize in aquatic therapy, and they should be able to give you specific recommendations.
Water aerobics is a great exercise, however if your husband has balance problems due to his Parkinson’s, he may benefit from participating in a balance and mobility exercise program that also includes resistance training to enhance his muscle strength and bone density. since you have knee problems the same program may also beneficial for you as well. I have the following suggestions: tai chi:moving for better balance and FallProof balance and mobility program.