Combine water, sugar, and mint in a small saucepan. Bring to a boil and boil until sugar is dissolved. Remove from heat and chill.
Place watermelon cubes in a blender or food processor and process until smooth. Pour through a fine mesh sieve to remove any solids. Chill the puree.
Combine rum, lime juice, 1 ½ - 2 cups watermelon puree, and mint simple syrup in a large pitcher. Pour into ice-filled glasses, garnish if desired.
