I recently came across the question above in the comment section.
My answer is scattered all over this blog, but I don’t think I’ve dedicated an entire post to the subject.
So here’s my opinion:
In fact, if I were to suggest a “Holy Trinity” of the most excellent knee exercises -- those where you can get in lots of nourishing high repetition movement with little strain on the joint -- it would look like this (in no particular order):
(1) easy walking (initially stay on flats, take breaks every 20 minutes or so, and don’t let your legs slam down -- ever)
(2) easy cycling (a stationary bike though more boring is better as it’s easier to control the force you exert; cycle backwards if your knees are really weak)
(3) easy water movement (careful -- swimming may be a very bad idea if you’re kicking vigorously with your legs; you’ll need exercises designed for people with tender knee joints)
The right kind of water exercises can be a great way to heal bad knees. Water provides gentle resistance and support when you're moving about. Of course just jumping in and doing the overhead crawl from one end of the pool to the other usually won’t be a smart idea.
Where do you find good exercises?
Heal Your Knees is a book that contains many water exercises designed for those with hurt knees. A frequent commenter here likes The Complete Waterpower Workout Book (both books share an author, who evidently has made water therapy a key part of her physical therapy treatments).
My biggest reservation about telling people to try to heal through water therapy:
Most of us simply aren’t close enough to pools, or can’t afford pool memberships, or can’t align our schedules with pool hours, to make this a really practical option. A nice thing about a walking regimen is that walking can be done anywhere, anytime.
The other thing to note: Water therapy may not be the best treatment plan for you, regardless of whether there's a convenient pool nearby. I did try it for a while and had some good results. But then I had a setback and found I wasn’t getting the same benefits as before, so I changed to a walking-centered program.