One 1-inch piece of fresh ginger, peeled and thinly sliced
Pinch of salt
2 cups seedless watermelon cubes
2 cups hulled strawberries
Ice and chilled sparkling water, for serving
In a small saucepan, combine the water with the sugar. Simmer over moderate heat until the sugar has dissolved, about 1 minute. Remove from the heat. Add the 2 mint sprigs, ginger and salt and let stand for 10 minutes.
Strain the syrup into a blender. Add the watermelon and strawberries and puree until smooth.
Fill 8 glasses with ice. Pour the agua fresca into the glasses and top off with sparkling water. Garnish with mint sprigs and serve.