Cut ripe watermelon into chunks and measure out 4 cups, removing seeds if necessary. Place the watermelon cubes into a blender.
Add a pinch of salt to the watermelon in the blender to enhance the flavor and create a balance between sweetness and saltiness.
Blend the watermelon on high speed until completely pureed, about 30-40 seconds. The mixture should be smooth and vibrant red.
Pour the watermelon puree into a freezer-safe container, spreading it out evenly.
If your freezer allows, set the container in the coldest part and let it freeze for about 2 hours, or until the edges start to firm up.
Remove the freezing mixture from the freezer, and use a fork to scrape and break up any ice crystals now forming. Stir to redistribute the slush.
Return the container to the freezer, repeating the scraping and stirring every 30 minutes for another 2-3 hours. Each time, the mixture will become more slushy and ice-like.
Once the mixture is evenly frozen and has a scoopable, icy consistency, give it a final stir and serve immediately.
