Step 1: Prepare your watermelon. If you haven't already, cut your seedless watermelon into 1-inch cubes. For the best, thickest, and coldest watermelon smoothie recipe, freeze these cubes for at least 2-4 hours, or overnight, until solid.
Step 2: Combine ingredients in a blender. Add the frozen watermelon cubes, freshly squeezed lime juice, mint leaves (if using), and 1/4 cup of cold water or coconut water to a high-speed blender.
Step 3: Blend until smooth. Start blending on a low speed, then gradually increase to high. Blend until the mixture is completely smooth and creamy. If the watermelon smoothie recipe is too thick, add more water or coconut water, 1 tablespoon at a time, until you reach your desired consistency. If your watermelon wasn't very sweet, now is the time to add optional honey or agave nectar.
Step 4: Taste and adjust. Give your watermelon smoothie recipe a quick taste. Adjust sweetness or tartness by adding more lime juice or sweetener if needed.
Step 5: Serve immediately. Pour the freshly blended watermelon smoothie recipe into chilled glasses. Garnish with an extra mint sprig or a lime wedge, if desired, and enjoy this perfect summer treat without delay!