Swimming kills my shoulders - I have probable EDS and my shoulders, wrists, and ankles are problems for me. Swimming has never been something I've been able to do for exercise, even as a kid.
Have you ever had a positive TTT? It is possible that you have something going on other than POTS or another form of dysautonomia. Have you monitored your HR at home and gotten orthostatic tachycardia? It is very frustrating to feel ill and not know the cause.