First, make the mint simple syrup: Combine 2 cups water, 1 cup sugar, and mint leaves in a medium saucepan and bring to a boil. Reduce heat and stir until sugar has dissolved. Once sugar has completely dissolved, remove from heat and allow mint to steep for 1 hour. Remove mint.

Next, cut the watermelon into small pieces which can fit in a food processor. (Remove and discard the rind). Process all the watermelon until it is a smoothie-like consistency. Strain into a pitcher. (Depending on the size of the watermelon, this could take a couple batches).

Finally, add 2 cups lime juice and mint simple syrup to watermelon juice and stir well. Serve over ice.