Step 1: Place the watermelon cubes and strawberries in a high-speed blender and process until completely smooth.
Step 2: If a smoother consistency is desired, strain the fruit puree through a fine-mesh sieve into a large pitcher; otherwise, pour directly into the pitcher.
Step 3: Add the lime juice and agave nectar to the fruit mixture and stir well to combine.
Step 4: In the bottom of each serving glass, place 3-4 mint leaves and muddle gently with a muddler or the back of a spoon to release the oils.
Step 5: Fill the glasses halfway with ice cubes, then pour the fruit mixture over the ice until the glasses are two-thirds full.
Step 6: Top off each glass with chilled sparkling water and stir very gently to incorporate.
Step 7: Garnish with a sprig of fresh mint and a small wedge of watermelon on the rim of the glass and serve immediately.