Put the cucumber, buttermilk, honey if using and lime juice into a blender. Blend on high until smooth. Add half of the frozen watermelon and blend until almost completely smooth, turning off the blender and pushing down on the ingredients with a spatula or wooden spoon as needed to help the blending process. Add the remaining frozen watermelon and continue to blend until smooth. Add 1 to 2 tablespoons of water if needed to adjust consistency. Pour into a glass and garnish with a cucumber slice.