Add the lime pulp to a pitcher and muddle gently to loosen the fibers and release juice. Add the mint leaves and muddle once or twice only, just until aromatic. Do not over-muddle — mint turns bitter and dark fast.
Add the watermelon cubes and sugar (if using) to a blender. Blend until completely smooth. Taste and adjust sweetness only if necessary — this should stay clean and light.
With the blender running on low, add the lime zest and blend briefly, just to incorporate. This preserves aroma without extracting bitterness.
Pour the blended watermelon into the pitcher with the lime and mint. Stir gently to combine — no aggressive mixing.
Fill glasses with ice, pour over the watermelon agua fresca, and garnish with fresh mint leaves. Serve immediately — mint should not sit in the drink.
