Add the watermelon to a blender and blend until smooth. Pour through a sieve to remove pulp.
Add the sugar, mint and water to a saucepan and heat at a medium high heat. Stir it constantly and when it starts to simmer, reduce the heat to low and cook for another 2 minutes. Allow to cool and remove the mint leaves.
Add the bourbon, watermelon juice and mint syrup together and shake.
Add some roughly crushed ice to your glasses and pour the cocktail over it. Then top up with club soda