In a food blender add watermelon, blitz on high for 30 seconds. You should have 2 cups of watermelon juice. Add the frozen lime concentrate, mint, spiced rum, vodka, and ice. Blitz on high for 30 seconds. Evenly divide daiquiris amongst 6 cups, garnish with a mint sprig, lime slice, frozen watermelon ball and straw, serve immediately.