2 cups cubed seeded watermelon
1 (1.5 fluid ounce) jigger vodka (such as Skyy®)
4 cubes ice, or as needed
1 dash lime juice (optional)
Blend watermelon in a blender until smooth; strain through a fine mesh strainer and discard pulp. Pour strained watermelon juice, and vodka into a cocktail shaker filled with ice; strain into a glass. Squeeze lime juice into the cocktail.