Tim Cook answered
Water on the knee is a not a condition in itself, but an everyday term which covers a range of problems that can affect the knee. Today, doctors refer to water on the knee as a knee effusion or bursitis.
How to Treat a Knee Effusion
Most cases of bursitis can be treated at home. Resting the affected area, using an ice pack (a frozen bag of vegetables wrapped in a tea towel works well) to reduce inflammation, and taking painkillers such as paracetamol and ibuprofen can help relieve your symptoms. If the swelling continues, and the afflicted knee reddens and becomes warm to the touch, then you will need to make an appointment with your physician. Here's a film showing some exercises you can do to strengthen the knee joints: