Cut watermelon into 1-inch chunks.
Add watermelon, lime juice, and honey to the blender.
Blend until smooth, about 30 seconds.
Pour mixture through a fine mesh strainer into a pitcher.
Add cold water and stir well.
Zest both limes using a microplane.
Add lime zest to the pitcher and mix gently.
Fill glasses with ice cubes.
Pour mocktail into glasses.
Top with sparkling water if desired.
Garnish with mint leaves and serve right away.