Drain lychees and add to a blender along with the watermelon, water and mint if using. Blend until smooth. (Depending on your type of blender you may have to strain the mixture with a fine sieve to remove any pulp). Pour into metal tray and freeze until partially set, about 3 hours. After 3 hours break up the ice that has formed with a fork and repeat this step every 30 minutes or so, for the next couple of hours until the granita has formed ice crystals
You could use any fruit in this recipe and if you like it a little sweeter add some sugar syrup to the mix.