Start by placing the cubed watermelon into a blender. Blend until smooth, then strain through a fine mesh sieve into a large pitcher to remove the pulp. Discard the pulp or save it for another use.
Add the freshly squeezed lime juice and honey (or agave syrup) to the watermelon juice. Stir well until the sweetener is fully dissolved.
Carefully add the chilled sparkling water to the pitcher. Gently stir to combine, making sure to keep the fizz intact.
Taste the mixture and adjust sweetness if necessary, adding more honey or agave syrup if desired.
To serve, fill glasses with ice and pour the sparkling watermelon limeade over the ice.
Garnish each glass with fresh mint leaves and a slice of lime on the rim for a refreshing look.
Notes
Adjust sweetness to taste with honey or agave syrup.