Is swelling of the knee and water on the knee a symptom of bone cancer or osteosarcoma?

Many reasons: There are many reasons for fluid collections in the knee or swelling of the knee. Bone cancer/osteosarcoma can cause this problem but it is much more likely to be another cause. Trauma will cause these symptoms as will arthritis or degenerative joint disease. This should be evaluated by your primary care physician.

Knee problem: It is most likely from osteoarthritis and very less likely from osteosarcoma.