Place watermelon chunks in a blender and blend until smooth.
Strain the watermelon juice through a fine-mesh strainer into a pitcher.
Add basil leaves to the pitcher and muddle gently with a wooden spoon to release the oils.
Pour in lime juice and stir to combine.
Add sparkling water and ice cubes right before serving.
Pour into tall glasses filled with ice.
Garnish each glass with fresh basil leaves and lime slices if desired.
Notes
The mocktail tastes best when served right away. The watermelon juice can be made up to 24 hours ahead and stored in the fridge.Fresh watermelon works better than frozen for this recipe. Pick a ripe, sweet watermelon for the best results.To make the drink sweeter, add 1-2 teaspoons of honey or agave nectar per glass.For a stronger basil flavor, let the mixture sit in the fridge for 30 minutes before adding the sparkling water.