Place the watermelon cubes into a blender. Blend on high until completely liquefied and pour through a fine-mesh sieve into a small bowl, pressing down on the pulp to extract all the juice. You should end up with approximately 2 ounces of juice.

Pour the salt onto a small plate. Run a lime wedge around the rim of a highball glass. Dip the wet rim into the salt.

In a cocktail shaker filled with ice, combine the tequila, watermelon juice, grapefruit juice, lime juice, and agave syrup. Shake well.

Strain into your prepared glass filled with ice. Top with club soda and garnish with a watermelon wedge.