Water exercise is fantastic and it can be used in so many ways. I recommend that you look into finding an instructor that knows how to teach aquatic exercise for all parameters of fitness. I.E. for range of motion, endurance, strength, cardio, etc.
And to add to Sue’s (above) answer, boxing without contact. No getting hit by someone else.
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.
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.