Prep your watermelon. Dice watermelon into 1-inch cubes. Place cubed watermelon in a single layer on a parchment paper lined baking sheet in the freezer and freeze for 5 hours, or until frozen.
Blend. Add frozen watermelon chunks, lime juice, and sea salt (if using) in a Vitamix or high-powered blender. If needed, wait a few minutes so it is easier to blend. Blend until watermelon is broken down into shaved ice consistency.
