if you cannot do any lower body exercises at all, you may need to look at an upper body ergometer, sometimes also called arm crank machine. Swimming is great but breast stroke usually needs to be avoided. Water walking can provide a good workout, and so does water aerobics.
Wish you good luck.
I saw one of your earlier questions was about Tough Mudder training. Do you still plan to do a race? Depending on your injury you will need to rehab your knee first and avoid any running for now (especially on uneven surfaces). Swimming is a great alternative and perhaps running/walking in the water will help with your cardio. Like Karin suggested, upper body ergometer could be an excellent alternative as well. Have you done any PT yet? Physical therapists can give you some great tips in getting your knee back in shape and work on any imbalances you might have. Muscle activation therapy can also be beneficial for you.
I hope this helps.
If you have a gym membership, try different cardio machines to get an idea of which machine(s) may be tolerable. Assuming you are treating the injured knee based on a medical diagnosis, then this Gould only be a temporary issue and you’ll soon have the mobility you once had to enjoy a wider range of cardio options.