Watermelon has a low glycemic index (GI), meaning it won't send your blood sugar on a rollercoaster ride. But it's the glycemic load (GL) that folks with diabetes need to keep an eye on.
For example, even though watermelon has a high GI ... Most adults with diabetes should eat 3 to 4 carb servings per meal and 1 carb serving per snack, though you should check with your doctor ...