Swimming is really good and unlike jogging, it doesn't do unspeakable things to your knees.

Just a note, but using your diaphragm is all well and good if you have nothing else trying to occupy said space! I made the interesting discovery recently that if I try to sing and use any of my abdominal muscles to help things along, I get paddled/kicked, or something (baby still too small for this to be clear, but sensations quite remarkable and very offputting.)Good posture does seem to help though, when I remember.