Light one chimney full of charcoal. When all charcoal is lit and covered with gray ash, pour out and spread the coals evenly over entire surface of coal grate. Set cooking grate in place, cover grill and allow to preheat for 5 minutes. Clean and oil the grilling grate. Brush each side of watermelon slices with spicy lime and honey mixture. Place watermelon and grill and cook until grill marks developed, about 2 minutes per side.