01 - Combine watermelon cubes, cucumber slices, mint leaves, and lime juice in a blender. Process until completely smooth and homogeneous.
02 - Pour blended mixture through a fine mesh sieve into a pitcher, pressing with a spatula to extract maximum juice while removing pulp for a smoother consistency.
03 - Stir in sweetener if using, mixing thoroughly until completely dissolved.
04 - Pour cold sparkling water into the pitcher, gently folding to incorporate while maintaining carbonation.
05 - Distribute immediately over ice in glasses. Garnish with fresh mint sprigs and lime wedges if desired.