Prep the watermelon: Cut the watermelon into 1-inch cubes. Lay the cubes in a single layer on a baking sheet lined with parchment. Freeze for at least 2 to 3 hours, or overnight for best results.
Load the blender: Add the frozen watermelon, 1 cup of ice, lime juice, sweetener (if using), and a pinch of salt to the blender.
If you like a minty note, toss in 3 to 4 mint leaves.
Blend low, then high: Start on low to help the blades catch. Increase to high and blend until smooth and thick. If it’s too thick or not moving, add a splash of cold water or a few more fresh watermelon cubes.
Taste and adjust: Check sweetness and acidity.
Add more lime for brightness, more sweetener if your melon isn’t very sweet, or a few ice cubes if you want it thicker.
Serve immediately: Pour into chilled glasses. Garnish with mint leaves and lime wedges if you like. Drink right away for the best slushie texture.