In a small saucepan, combine the mint, sugar and water. Heat over medium low heat until the mixture reaches a slight simmer. Turn the heat off and allow the mixture to steep for 1 hour. Strain, discarding the mint leaves, and transfer to a small bowl. Set aside.
Over a hot charcoal grill (or alternatively you could use a griddle - this one is my favorite), grill the wedges of watermelon for about 1 minute per side. You'll know they're done when they reach a dark red color and have visible grill marks. Transfer them to a plate and allow to cool slightly. When cool enough to handle, chop up the watermelon into cubes, discarding the rind. Transfer the watermelon to the jar of a food processor or blender and pulse until pureed, about 10 seconds. Run the watermelon puree through a strainer and discard the pulp.
Pack your cocktail glasses with crushed ice. Divide the bourbon between the glasses. Next, add a tablespoon or two the mint simple syrup and then top with the watermelon puree. Garnish with mint leaves.