To prepare the watermelon puree, combine chunks of ripe seedless watermelon, simple syrup and freshly squeezed lime juice in a blender. Process until smooth.
In a Boston shaker, combine Arrogante Reposado tequila, pureed watermelon, simple syrup, St-Germain Elderflower liqueur and freshly squeezed lime juice. Add a handful of ice cubes and shake vigorously for 15 to 20 seconds.
Strain the drink in a rocks glass filled with large pieces of ice. Garnish the cocktail with a pinch of finely diced watermelon rind.