In a food processor or blender, puree the watermelon, cucumber, sugar, and lime juice until completely smooth. Pour the mixture into a 9 x 9-inch metal baking pan.
Freeze for about 1 1/2 hours until it's nearly set, then using a fork, give it a good scrape to mix everything around and break up any chunks. Return the pan to the freezer and freeze until almost set again, 1 to 2 hours.
Use a fork to scrape the granita to form chunky snow-like ice crystals. Freeze and repeat occasional scraping until the entire mixture is frozen and shaved, about 5 hours total.
To serve, divide the granita among serving glasses, and garnish with cucumber if desired.