Place the watermelon and lime juice in a blender. Process until smooth.
Using a fine sieve, strain the watermelon and lime juice into a small pitcher or cocktail shaker.
Using a wooden spoon, press any extra juice out of the pulp through the mesh of the sieve. Set the pitcher aside.
Place 4 mint leaves at the bottom of each of four 8 to 10-ounce old-fashioned cocktail glasses. Use a muddler to smash the mint and release the oils and herbal aroma.
Fill each glass with ice cubes. Pour the watermelon/lime juice over the ice to fill the glasses 1/2 way. Top each glass with sparkling water.
Garnish each mocktail with a lime slice and a mint leaf. Serve immediately and enjoy.