01 - Place cubed watermelon in a blender and process until completely smooth, forming a vibrant puree.
02 - Pour watermelon puree through a fine-mesh sieve into a large pitcher to remove fibrous pulp, pressing with a spoon to extract all liquid. Skip if you prefer a thicker texture.
03 - Add freshly squeezed lemon juice and sugar or agave syrup to the pitcher. Stir vigorously until sweetener fully dissolves into the fruit mixture.
04 - Pour in cold water and mix thoroughly. Taste the beverage and adjust sweetness by adding more sweetener or increase tartness with extra lemon juice as desired.
05 - Refrigerate for at least 1 hour to allow flavors to meld, or serve immediately over ice cubes in tall glasses.
06 - Decorate each glass with fresh lemon slices and mint sprigs before serving for an elegant presentation.