Cut the watermelon into chunks removing the rind and seeds. Place the chunks into a bowl and let them cool in refrigerator for 1-2 hours.
In blender, combine watermelon, lime juice, and simple syrup, and puree. If you prefer to remove the pulp, strain through a fine sieve. You will also need to double the portions for each ingredient per glass.
Pour into the glass and garnish with a slice of lime and mint leaves (optional). Chill before serving if you like.