Start by cutting the watermelon into small cubes.

Add the watermelon cubes to your blender. Blend on high until completely smooth, which usually takes about 30 seconds.

If you prefer a smoother texture, pour the watermelon puree through a fine mesh strainer into a pitcher.

Juice the limes to get about half a cup of fresh lime juice. Pour this juice into the pitcher with the watermelon puree.

Add the honey or agave syrup to the pitcher. Stir well to dissolve.

Pour in the cold water or sparkling water. Stir gently until everything is combined.

Let the limeade chill in the refrigerator for about 30 minutes.

Fill serving glasses with ice cubes and pour the watermelon limeade over the top. Garnish with fresh mint leaves if desired.