To make the watermelon juice, in a blender, puree 2 cups fresh watermelon cubes on high for 1 minute.
If desired, wet the rims of 2 glasses with lime juice and dip in coarse salt. Fill the cups to the top with ice.
Add the watermelon juice, tequila, lime juice, and maple syrup to a cocktail shaker. Add 3 ice cubes and secure the lid or top cup. Shake vigorously for 30 seconds, until chilled.
Strain the cocktail into the prepared glasses and garnish with a slice of lime.
