Make the Simple Syrup
Stir the lemon zest, sugar, mint leaves, and water in a saucepan over medium-high heat.
Bring to a boil, then turn the temperature to medium/medium low to simmer for about 10-15 minutes, or until the color of the syrup turns yellow from the lemons, the sugar has completely dissolved, and the syrup is fragrant.
Turn the heat off, and pour the lemon mint syrup through a mesh sieve into a heat-safe container.
Make the Watermelon Lemonade
Purée the watermelon flesh in the blender until smooth. Pour it through a fine mesh sieve into a pitcher to remove the pulp.
Add the lemon syrup, lemon juice, and water to the pitcher and stir until well combined.
Chill in the fridge covered with plastic wrap or a lid until ready to serve.
Serve over ice cubes or frozen watermelon cubes.