[Sundae]

Consider aqua aerobics/ water workout too.
You burn calories, tone muscles and if you have an even halfway decent tutor they will check on any existing injuries/ weaknesses at the start of every class and tell you when you need to amend the exercise to suit. I know it's not as easy as swimming in your apartment pool, but once you know what you are doing there's nothing to stop you buying the same floats etc as the class uses and doing your own resitance work alone.

Swimming is great exercise of course, but I'd get advice from someone qualified if you have a specific aim in mind. Straightforward swimming might not meet it, in the same way going all out on a treadmill doesn't.