Taping to limit some rotation at the knee may help. Have tried this with a client that had significant OA in her knee and seemed to help...
Basically taped a figure-8 around the knee - one each way. (Starting medial thigh (i.e. just above the knee) - crossing knee joint at lateral joint line - behind knee to medial "calf" (i.e. just below the knee) - around front of tibia - back behind the knee back to starting point; then do the same thing the opposite way (i.e. start on lateral side of "thigh"))

We used a good quality vet wrap (stronger than coban (?brand name) made for humans). Even typical vet wrap found in pet stores / livestock supply stores would probably work well as it probably will stand up to the water better than coban made for humans.

Not sure if it will work as well in an non-weight bearing position as with swimming but seems to be somewhat helpful in weight-bearing. Worth a try??