To a cocktail shaker, add the juice of ½ lemon, ½ oz simple syrup, 1 cup roughly chopped seedless watermelon flesh and 1½ oz gin. Add as much ice as will fit, shake sharply and strain into a Collins glass filled with ice. Garnish with a wedge of watermelon.
To a blender, add 2 cups water and 5 cups chopped seedless watermelon. Blend until finely puréed and pour through a fine-mesh strainer into a large pitcher. Add ½ cup freshly squeezed lemon juice, ¾ cup simple syrup and 1 cup gin. Stir to combine and serve over ice garnished with a wedge of watermelon.
Recipe very slightly adapted from: St. Onge, Christopher. "Watermelon Squeeze." Food and Drink. Summer 2011.