Add watermelon cubes to a blender or food processor and puree until smooth. Use a fine mesh strainer to strain to mixture into a cocktail shaker filled with ice.
Add grapefruit juice, tequila, and lime juice to the shaker. Shake well for about 15 seconds.
Strain into two cocktail glasses filled with ice. Top with club soda.
Garnish with watermelon wedges dipped in coarse salt.
notes
If you like extra salt, you can rim the glasses with salt as well!