Prep the watermelon – Cut the watermelon into chunks. Remove any seeds. Measure about 4 cups.
Blend – Add watermelon, sugar, lime juice, and mint (if using) to a blender. Blend until smooth and liquid.
Freeze – Pour the mixture into a shallow baking dish. Place in the freezer for 1 hour.
Scrape – After an hour, use a fork to scrape the mixture into icy crystals. Return to the freezer.
Repeat – Continue scraping every 30–45 minutes for about 3 hours, until fluffy and icy.
Serve – Spoon into chilled glasses. Garnish with mint leaves or a wedge of watermelon.