Use a cutting board to roughly chop the mint leaves to release their aroma. Add the fresh mint leaves to the blender.
Pour the watermelon cubes into the blender, along with chilled water. If desired, add honey for sweetness.
Secure the lid on the blender and blend on high speed until the mixture is well combined and slightly frothy, about 30-45 seconds. Observe the color turning pale pink with specks of green from the mint.
Stop the blender and check the texture; it should be smooth with tiny seed fragments visible, giving a slightly gritty feel when poured.
Pour the smoothie into chilled glasses, ensuring some small seed particles remain suspended for texture. Serve immediately garnished with a sprig of mint if desired.