Well.. this is all I could find. I couldn't find anything relating to a cure. But it seems as though it definitely has a positive effect for diabetes ..
Hope it helps.. it seems as though it might be worth having quite a bit of cress in the diet..
http://juicing-for-health.com/basic-nutrition/healing-vegetables/health-benefits-of-watercress.html
Blood glucose: Although not definitely proven, it seems that watercress can help control blood sugar levels and prevent or treat high blood glucose. It is probable that the hypoglycemic effect of watercress is in part due to its high content in soluble fiber, that helps reduce the absorption of carbohydrates from the intestine. This effect is particularly helpful for individuals with diabetes.
http://www.diethealthclub.com/health-food/watercress-health-benefits.html
This low fat, low sodium, low cholesterol food has plenty of benefits even for patients suffering from diabetes. Watercress for diabetics works because it has a positive effect on the pancreas, helping to augment blood sugar levels and maintaining insulin production. Since obesity is the number one cause of diabetes, watercress for diabetics helps to maintain weight loss. Plenty of essential vitamins and minerals and meager calories makes watercress an ideal choice of food for diabetics. Its culinary versatility and flavor make it a cooking favorite. It has replaced traditional herbs and now appears alongside main course dishes. Watercress contains complex carbohydrates, which considerably reduce the amount of insulin needed and regulates the blood sugar levels, which may spike after mealtimes. Experts suggest including at least 3 to 4 portions of watercress in your diet per week to extract maximum benefits.
http://completewellbeing.com/article/garden-cress-packed-with-power/
For diabetes
The seed coat of germinating seeds contains mucilage, which has a phytochemical called lepidimoide. Studies show that seeds of the plant lower the glycemic response to a test meal.