Halve a ripe seedless watermelon and scoop out rounded Tablespoon amounts (or cube with a knife). Arrange on a parchment-lined baking sheet. Freeze until firm - about 3-4 hours.
Once watermelon is frozen, add ~ 4 cups (amount as original recipe is written // adjust if altering batch size) frozen watermelon (I found 4 cups was about three-quarters of one small watermelon), coconut milk, lime juice, and watermelon juice to a blender and blend on low until creamy and smooth. You will need to scrape down the sides and blend on low to encourage it along. If it has trouble blending, add a bit more watermelon juice, but not too much or it will become too thin.
Once the slushie is blended and smooth, taste and adjust flavor as needed. Add more coconut milk for creaminess, lime for brightness, a pinch of salt to enhance sweetness, or sweetener of choice to sweeten. Blend and sample again - repeat as needed.
*Nutrition information is a rough estimate.