How to Make It
In a small saucepan, combine the water and sugar and bring just to a boil, stirring until the sugar is completely dissolved. Transfer the simple syrup to a heatproof bowl and let cool.
In a food processor or blender, puree the watermelon in batches until smooth. Strain the puree into a large bowl or pitcher. Stir in the simple syrup, tequila, lime slices, 3 mint sprigs and 1/2 cup of water.
To serve, pour the watermelon agua fresca into ice-filled glasses, leaving 1 inch at the top. Top off the drinks with Prosecco and garnish with mint leaves.