Begin by placing the cubed watermelon into a high-speed blender. Blend the watermelon until it reaches a smooth consistency. To eliminate any pulp, strain the blended mixture through a fine mesh sieve into a large pitcher. You may discard the pulp or keep it for a creative use later.
Next, pour the freshly squeezed lime juice into the pitcher with the strained watermelon juice. Add the honey or agave syrup and stir thoroughly until the sweetener is completely dissolved and integrated into the juice.
Gently introduce the chilled sparkling water into the mixture in the pitcher. Use a soft stirring motion to combine the ingredients without disrupting the carbonation.
Taste your refreshing blend and adjust the sweetness to your liking. If desired, add a touch more honey or agave syrup and stir lightly to incorporate.
For serving, fill individual glasses with ice cubes. Pour the sparkling watermelon limeade over the ice to ensure a refreshing chill.
To enhance the presentation, garnish each glass with a few fresh mint leaves and a vibrant lime slice perched on the rim.