Combine all of the ingredients, except the oysters, in a small saucepan and bring to a simmer over medium heat. Stir until all of the sugar and salt have dissolved.
Pour the mixture into a wide flat dish and put it in the freezer. As the mixture begins freeze, drag a fork through it to break up the ice crystals. Repeat this process every 15 minutes or so until the mixture is completely frozen. When done it will look like ice shards, kind of slushy but a bit more frozen.
Scrub the outside of the oysters to remove any sand and grit. If using, spread the rock salt out on a serving dish. Shuck the oysters and lay them on the rock salt. Top each oyster with the granita and serve immediately.