Prepare the watermelon by cutting it into small cubes and measuring two cups of the pieces.
2 cups fresh watermelon cubes
Add the watermelon cubes to a blender along with lime juice and orange liqueur if using. Blend on high speed until completely smooth and slightly frothy, about 30 seconds.
2 cups fresh watermelon cubes, 1 oz lime juice, 0.5 oz orange liqueur
Strain the blended mixture through a fine-mesh sieve into a bowl or pitcher, pressing gently to extract the liquid while leaving seeds behind, resulting in a smooth, slightly grainy pulp.
2 cups fresh watermelon cubes
Pour the strained watermelon liquid into a shaker filled with ice. Add tequila and shake vigorously for about 15 seconds until well chilled.
3 oz tequila
Prepare cocktail glasses by rimming the edges with salt. Strain the margarita into the glasses and garnish with a small watermelon wedge or lime slice if desired. Serve immediately, chilled.