In a blender, blend the lime juice and watermelon together, creating watermelon water. Strain the liquid through a fine mesh strainer to remove any seeds. Place 1/2 cup of the watermelon water into a saucepan and add the sugar, lime zest and ginger coins. Bring the mixture to a simmer to dissolve the sugar and infuse the liquid with the ginger and lime flavor. Set aside to cool. Add the grated ginger to the remaining watermelon water. Once the syrup is cool, remove the ginger coins and mix into the watermelon water. Pour the mixture into a 9 by 13-inch glass baking dish. Place the dish into the freezer to chill. Every 30 minutes, scrape the mixture with a fork to break up the ice crystals and to keep it light and fluffy. Continue scraping every 30 minutes for at least 2 hours. Scrape the granita into elegant glasses and sprinkle with fleur de sel.
Taste your watermelon, this will tell you how much sugar to add to your simple syrup. The fork helps to create ice crystals that will help give you a slushy consistency.