Combine water and sugar in a saucepan.

Bring to a boil over medium-high heat, stirring until sugar dissolves (about 30 seconds).

Remove from heat and stir in mint, lime rind, and lemon rind.

Let the mixture stand for 20 minutes to infuse the flavors.

Strain the mixture through a fine mesh sieve, discarding the solids.

Place one-third of the sugar syrup and watermelon in a blender.

Process until smooth.

Pour the puree into a large pitcher.

Repeat the blending process twice with the remaining sugar syrup and watermelon.

Stir in lime juice and lemon juice.

Serve over ice or refrigerate until ready to serve.

Stir before serving.