1. Puree all ingredients in a blender until super smooth, and then strain through a fine sieve or chinois. Then place in a large metal baking pan about 9 x 9 x 2 that will allow the granita to freeze quickly and evenly.
2. As the watermelon begins to freeze, take a fork and rake the liquid about every 30 minutes over a span of 3 or so hours. Before serving, rake the ice one more time with the fork and serve immediately or store in the freezer for later use.

For plating:

Serve the watermelon granita over top of the lime posset and garnish with chocolate that you can pipe into dots or in the shape of watermelon seeds.