Place the watermelon cubes into a large blender and blend until smooth, about 30 seconds, until the mixture is vibrant pink and slightly frothy.
Pour the blended watermelon through a fine mesh strainer into a pitcher or bowl, pressing gently with a spatula to extract as much juice as possible. Discard the pulp leftover in the strainer.
Add freshly squeezed lime juice to the strained watermelon juice, whisking gently to combine. Stir in honey or simple syrup if you want a touch of extra sweetness, adjusting to taste.
Lightly bruise the mint leaves with the back of a spoon to release their aromatic oils, then add them into the mixture. Let it sit for about 5 minutes to infuse the flavor.
Remove the mint leaves from the liquid, then pour the Agua Fresca into glasses filled with ice. Garnish with a sprig or a few leaves of fresh mint for a bright presentation.
Serve immediately with a straw or a slice of lime, enjoying the cool, crisp refreshment that highlights the sweetness of the watermelon and the freshness of the mint.