Start by cutting the watermelon into cubes, measuring out 4 cups.
Peel and dice the cucumber, measuring out 1 cup of diced cucumber pieces.
In a blender, combine the cubed watermelon and diced cucumber.
Add the fresh lime juice, honey or agave syrup (if using), and cold water to the blender.
Blend on high speed until the mixture is smooth.
Place a strainer or fine mesh sieve over a pitcher and pour the blended mixture through to separate the juice from the pulp.
Use a spoon to press down on the pulp, extracting as much liquid as possible.
Discard the pulp and refrigerate the juice for a few minutes or serve immediately over ice.
Garnish with fresh mint leaves, if desired, and enjoy your refreshing cooler!