Do you have a pacemaker? And is it on that side? Does it get red at any other time in addition to swimming? Did you have any vascular work done in that area during your heart valve surgery or during the pacemaker implantation?
Maybe the movement during water aerobics is irritating the pacemaker pocket. Or maybe more blood is moving into your arm during the exercise and somehow can't circulate properly back yo where it needs to go. Does it swell at all?.
I think you should ask your doctor to rethink his investigation of this problem. Maybe it is nothing, or maybe it is something. But until you have a definite answer, you won't know which it is.
If your cardiologist won't help you, then see your Internist and/or a vascular doctor.
Something is going on. You need answers, not the big brush-off.