1. Slice a cucumber into thin slices, add them to a sealed container with roughly 1 cup of simple syrup (1:1 sugar and water), and let infuse overnight in a refridgerator.
2. Juice a few slices of watermelon and strain with a fine mesh strainer.
3. Add the gin, juice, syrup and lime juice to a cocktail shaker and shake with ice.
4. Top with club soda and garnish with a slice of cucumber.