Heat a gas or charcoal grill to high heat (about 450˚ to 500˚) and oil the grill grates with vegetable oil.  In a small bowl, stir together the sugar, salt, and lime zest. Pinch the mixture together with your fingertips to combine all of the ingredients. Lay the watermelon wedges in a single layer on a sheet tray. Reserve 1 teaspoon of the salt mixture; set aside. Sprinkle half of the remaining salt mixture all over the watermelon wedges. Flip and sprinkle the wedges with the other half of the salt mixture. Place on a cooling rack set over a sheet tray and let rest for 15 minutes. Using paper towels, press firmly on the slices to remove excess moisture. Place the wedges on the grill over direct heat and cook for 2 to 3 minutes on each side, until grill marks appear. Remove the watermelon from the grill and place on a sheet tray or serving platter. Sprinkle with the reserved salt mixture and cilantro. Serve with lime wedges, if desired.