You can take a water aerobics classes and/or do your walking in a swimming pool (with plenty of other people who aren’t exactly fond of wearing swimsuits), or you can use a walker.
The main idea is to start where you are right now, and adapt exercises to your needs and capacities, instead of trying (and often failing) to use exercises that aren’t right for you at this stage. With a little research and by asking questions, you’ll find that plenty of very effective alternatives to traditional exercises are already available. That’s why we have a Fitness Resource Center, Resident Experts, and the Community Message Boards, where you can get support and tips from lots of people struggling with the same problems you're facing.|
Above all, don’t make it easy to talk yourself out of starting an exercise program by getting confused about the difference between a challenge and an insurmountable obstacle. Those undefeatable obstacles are really pretty few and far between and not so hard to work around—if you want it to be that way.