Combine all ingredients except rock salt and oysters, pour into a loaf pan (to a depth of about 1 in.), and freeze 1 hour. Rake and crush mixture with the tines of a fork and freeze for another hour. Rake and crush mixture again and freeze 1 hour more.
Spread rock salt on a platter. Rake and crush granita a final time into fine crystals. Set oysters in place and top each with a spoonful of granita.
Note: Nutritional analysis is per serving.