Blend together 4 cups watermelon with 1 cup kiwi and ½ cup ice until smooth. Strain the juice if desired.

Combine ¼ cup granulated sugar with the zest of 1 lime in a small bowl. Wet the rim of the glasses with a lime wedge and dip in the lime sugar. Add 2 slices of lime and 2 mint leaves per glass. Muddle until the mint and lime are slightly bruised and pour the watermelon kiwi agua into the glasses. Serve cold.