In general, a healthy body weight is best for your spine. Even losing 5 pounds can exponentially decrease the weight through your spine. There are not any specific diets proven to help degenerative disc disease (DDD) and supplements are debatable. You can always ask your physician about specific supplements and herbs as they may interact with your current medications.
Exercise in general is very good and an important part to healing. Pool exercise is a great way to get in shape as the weight through your spine is decreased. Walking and weights in the water are good daily exercise choices. Stretching and abdominal strengthening may also be very important, but an evaluation from a physical therapist would be best to determine your specific areas of weakness, stiffness, lack of flexibility, and directional preference. A good general rule is to exercise daily doing something that does not cause pain during or after. Over time progress in a slow, linear fashion.