Add all of the juice ingredients (cubed watermelon, lime juice, honey, and salt) to a blender and blend on medium for about 30 seconds until smooth.
Strain the mixture in a large bowl through a fine mesh sieve, then pour into a serving glass.
Make the cocktail:
Add the cucumber slices and mint leaves to a cocktail shaker. Muddle until the cucumber gives little to no resistance.
Add the watermelon juice and gin to the cocktail shaker along with a scoop of ice. Shake vigorously.
Prepare the glass by mixing the lime zest and salt in a low dish, running a lime wedge over the rim of your glass, and rolling the rim in the zest and salt mixture. Add ice.
Pour the drink over ice and garnish with a watermelon wedge and a sprig of mint. Cheers!