You might want to check anything you do with your specialist before you do it but if you can get hold of a swimming book called Total Immersion. It has good shoulder exercises meant to prevent injury in shoulders which are common in swimmers also. My swimming coach is all for it and she studies human movement so I'd like to think her opinion is reliable.

Still I'm not sure when you would do these exercises - I'm not sure if it's safe to do them before your operation or not. Sorry i couldn't be more help. I've had to reduce my training due to a knee injury for only 6 months so far and I hate it so you must be hating it a lot more after 2 years!