I do deep-end water aerobics (called hydrofit at my gym) twice a week, and really enjoy it. I've found that you get out of it what you put into it so if you're willing to work hard and move, then you will get a good cardio workout as well as resistance. We have a great instructor, which helps a lot.
I do recommend doing land aerobics and/or lifting weights to get the kind of workout you need to keep your bones healthy.
Women who behave rarely make history.