As a 56 year old with a challenge knee I can relate. Walking isn't a bad alternative, doing some hiking with trekking poles. I agree on the water, I love deep water exercise. You do wear a flotation device so wonder if that would help with your fear. Really helped me after a bad couple years with achille's issues and a broken foot. I am trying a boot camp for the first time ever - so far so good, I major modify and the trainer has been great about giving me alternatives to what the rest of the group is doing. Keep up the good work. Being healthy and feeling good means way more to me than what ever the scale might announce