01 - Add the frozen watermelon cubes, coconut milk, lime juice, and agave syrup to a high-speed blender.
02 - Blend on high until completely smooth and slushy, scraping down the sides as needed to ensure even consistency.
03 - Taste the mixture and add more agave or honey if desired, blending briefly to incorporate.
04 - Pour into chilled glasses, garnish with lime slices and fresh mint if using, and serve right away.