Cut the watermelon into roughly 2-inch cubes using a sharp knife and cutting board, removing any seeds if necessary.
Arrange the watermelon cubes in a single layer on a baking sheet lined with parchment paper to prevent sticking.
Place the baking sheet in the freezer and freeze the watermelon chunks for at least 4 hours or until completely firm.
Remove the frozen watermelon from the freezer. Use a fork to scrape the ice and flesh into a fluffy, granular texture, resembling snow or crushed ice.
Continue scraping until the entire batch is broken into small, snow-like flakes. Serve immediately for a soft, icy texture or transfer to an airtight container and refreeze for firmer consistency.