Place the watermelon cubes into a resealable plastic bag. Sprinkle in the tequila and seal. Refrigerate while you’re making the guacamole.
Cut the avocados in half, remove the pit then scoop the flesh from each half into a large bowl. With an old-fashioned potato masher, a large fork or the back of a large spoon, coarsely mash the avocado.
Grate the zest from half of the lime into the avocado. Juice the lime and squeeze in 2 tablespoons of juice.
Finely grate the ginger. Measure about 1/2 teaspoon and add it to the avocado, along with the chile and herbs. Stir to combine, then taste and season with salt, usually about 1 teaspoon. Cover with plastic wrap pressed directly on the surface of the guacamole and refrigerate until serving time.