First, you absolutely should talk to your surgeon or general practitioner.
Yes, swimming is very low impact. So low in fact that it could be called "no impact." Water resistance on all parts of the body while swimming means that many muscles will be worked. Because we are largely buoyant in water stresses on joints and ligaments are greatly reduced.
Bicycling is higher impact than swimming, but considerably lower than running, walking or jogging. After all, your leg motions are circular with no sudden starting and stopping at the joints.
Yoga is also low impact, but requires a wide variety of "poses" some of which could impact that knee. Should you consider yoga consult your physician and the yoga-master.
Many pilates routines are also low impact, but not all. As with yoga I'd speak to your physician and an expert instructor before starting.
The most important step is speaking to your health professional and explaining that you need to exercise.
Good luck. I hope you have a happy outcome and are once again the picture of good health.