Medically, there is no reason you can't do 'regular' water aerobics if you're healthy, though it's possible you might want to look for a 'light' water aerobics class. It appears a number of pools in Seattle offer water aerobics. If being indoors is an issue, the West Seattle Family YMCA might be an option; their phone number is (206)935-6000. Another specific option in a different part of town might be the Columbia Athletic Club; phone contact is (425)821-0882.