In a medium saucepan over medium heat, combine fresh basil leaves, sugar, and water. Stir gently for about 5-10 minutes until sugar dissolves. Remove from heat and let cool.

Strain the cooled syrup using a fine mesh strainer to remove basil leaves, setting aside the syrup.

Blend watermelon chunks until smooth, then strain through a fine mesh strainer into a bowl, discarding pulp.

In a cocktail shaker, combine watermelon juice, basil syrup, and lime juice with ice. Shake vigorously for 15-20 seconds.

Fill tall glasses with ice and strain the mixture over. Top with sparkling water and garnish with basil leaves and lime slices.