To start, pour sugar and water into a shallow sauce pan and bring to a simmer over a medium heat. Stir until sugar has dissolved and refrigerate for an hour. *
Add mint leaves, lime juice, simple syrup, and watermelon to the bottom of a tall glass, use a muddler or the back of a wooden spoon to muddle. **
Add rum and fill glass two-thirds of the glass with ice. Top off with soda water and stir well using a bartender's spoon.
Garnish with a sprig of mint.
*You can make this in advance so that you don't have to make it on the spot. Alternatively, you could also buy it at the store.**I left the watermelon chunks inside but if you prefer, you can muddle the watermelon separately and filter out any pulp.