In a small saucepan, combine the water, sugar, and mint leaves.
Bring the mixture to a boil and cook over medium-high heat, stirring occasionally, until the sugar fully dissolves.
Remove from the heat and let the mint steep for 30 minutes.
Strain the syrup through a fine-mesh sieve, pressing on the mint leaves to extract as much flavor as possible. Allow the syrup to cool completely before using.
In a blender, combine the watermelon cubes, lime juice, ¼ cup mint leaves, and ¼ cup of the prepared mint simple syrup.
Blend until smooth.
Add ice cubes to a cocktail shaker and pour the blended mixture over the ice.
Add the tequila.
Shake vigorously.
Strain the mixture into glasses filled with fresh ice.
Serve, garnished with lime slices and extra mint leaves.