This low calorie watermelon martini recipe is just 119 calories!You can use fresh or frozen watermelon to make this low calorie cocktail. And if your watermelon is not that sweet, freezing it first will help with that!If you don't have a cocktail shaker or muddler, you can blend the watermelon and then strain it. You can also muddle it with a spoon, then strain into a glass.