In reply to SiobhanStraver:
When your leg gets better, use of a cross trainer in a gym is good for cardio and low impact on the knees. Cross trainers with cyclical leg action, as opposed to just the feet sliding up and down a rail, i prefer, and cross trainers don't have the arms overhead activity that swimming and climbing share, possibly putting you on a path to Shoulder Impingement Syndrome.
But if you are just swimming whilst off climbing, it could be ok.
I may try some swimming, see how it affects my sis, but thought it wasn't supposed to be ideal for climbers, except we have climbers posting on this thread recommending it, so may be worth a go.