Put the watermelon cubes and salt in the blender and blend until smooth.
Fill the glasses with ice, and divide the mixture between the 4 glasses.
Squeeze a lime quarter into each glass before dropping it in for garnish. Serve right away.
Instead of cutting the lime into quarters and using it as a garnish, use a grater or zester to scrape its peel into the blender, then cut it in half and squeeze in the juice. It will be much limier this way!