In a small saucepan, bring the water and sugar to a simmer. Stir until the sugar is dissolved, about 1 minute; let the sugar syrup cool.
In a blender, puree the watermelon until smooth. Strain the watermelon juice, pressing gently on the pulps to extract as much juice as possible. Discard the pulp.
In a pitcher, combine the sugar syrup with the lime juice, calamansi juice and mint leaves. Using a wooden spoon, lightly muddle the mint. Add the watermelon juice and tequila.
Add ice to the cocktail and
garnish with the mint sprigs and serve cold.