1. Make the Panna Cotta: In a large saucepan, heat the milk with the sugar over low heat, stirring, until the sugar is dissolved. Meanwhile, in a small bowl, sprinkle the gelatin over the water and let stand until softened, 3 minutes. Scrape the gelatin into the warm milk and let cool completely.
2. In a medium bowl, whisk the sour cream with the yogurt, lime juice and vanilla. Whisk in the milk and pour the panna cotta into 6 glasses. Refrigerate until set, 3 hours.
3. Meanwhile, Make the Granita: In a saucepan, cook the water and sugar over moderate heat, stirring, just until the sugar is dissolved. Let cool completely.
4. Cut three-fourths of the pineapple into chunks, transfer to a blender and puree; strain into the saucepan with the sugar syrup, pressing hard on the solids. Discard the pulp. Stir the salt into the pineapple puree and pour it into a 9-by-13-inch baking dish. Freeze for 30 minutes. Using a fork, stir the granita. Continue freezing, stirring every 30 minutes, until the granita is icy and fluffy, about 3 hours.
5. Cut the remaining pineapple into thin slices. Fluff the granita with a fork and spoon it over the panna cottas. Garnish with the pineapple slices and serve right away.