想做帶氧運動, 但膝部有點問題, 不宜做撞擊性的運動, 跑步跳舞都不宜, 除左游水, 仲有咩帶氧運動可以做呢?

The best cardio for you is swimming, if you don't want to swim, try to join some water aerobic, the water will help the pressure on your knee, hopefully with help your knee stronger.
Another exercise is pilates (but it is not really cardio), many atheltes and people with injuries use pilates to keep exercises while they are still recovering; most pilates move are on a mat, make your abs. muscle getting stronger and more flexible.
The most important is ask your doctor or physical therpy about your exercise plan, and knowing your limit, don't push it, you don't want to make it worse.