Puree a medium-sized watermelon in a food processor and strain enough to equal one cup of juice. In a small sauce pan, heat the sugar and watermelon juice until the sugar dissolves. Remove from heat. (Mixture can keep in the refrigerator for a few weeks.) In a cocktail shaker with ice, add Pimm's lemon, and 1 ounce of the watermelon syrup. Strain into a Collins class filled with ice and top with ginger ale or lemon-lime soda. Garnish with cucumber slices.