Blend the chilled watermelon in a blender and strain for a smoother cocktail to serve on ice. For a frozen variation, freeze some watermelon cubes and blend it in with the chilled watermelon.
In a large pitcher, muddle 1 lime, herb leaves, and 1-2 jalapeños. Use a rolling pin or the bottom of a glass bottle if you don't have a muddler.
Stir in the blended watermelon, liquor, a splash of soda, and sweeten to taste with simple syrup.
Serve in ice-filled glasses and garnish with lime wedges, herbs, and jalapeño slices.
For a non-alcoholic version, simply leave out the liquor and enjoy!