Prepare the watermelon juice: Add the cubed watermelon to a blender and process until smooth. Then, strain the mixture through a fine mesh strainer to remove pulp and seeds.
Muddle the basil: In the bottom of a pitcher or large glass, gently muddle the basil leaves to release their oils and flavor.
Mix the base: Add the strained watermelon juice, lime juice, and honey to the muddled basil. Stir until the honey dissolves completely.
Chill the mixture: Refrigerate for at least 30 minutes to allow flavors to blend. This step can be done up to 24 hours in advance.
Serve the mocktail: Fill glasses with ice cubes. Then, pour the watermelon-basil mixture over ice, filling the glasses about 3/4 full.
Add fizz: Top each glass with sparkling water and stir gently.
Garnish: Add a sprig of fresh basil and a lime slice to each glass before serving.