This recipe is based on taste. Every watermelon has its own level of natural sweetness. Therefore, use it as a guide. The measurements don't need to be exact. Puree the watermelon adding the lime juice and then sweeten with the sugar from there. You may not need it all.
Puree long enough to be sure that the sugar is completely dissolved in the liquid.
Pour into a loaf or square pan. Glass is best as you will be scraping it every 2 hours until the crystals are formed. You don't want to scrape your metal pans!
Cover with plastic wrap and freeze. Once you have achieved the ice crystals, you can freeze for up to a week but may need to thaw a bit and scrape again before serving.
Serve with a slice of watermelon and a wedge of lime for garnish, if desired.