In a medium saucepan, combine the raspberries with the water and cook over moderately high heat, crushing the berries with a potato masher or a fork, until very juicy, 5 to 6 minutes. Pour the hot berries into a fine stainless-steel sieve set over a medium bowl and let drain.
Gently stir the berry mixture, scraping it up from the bottom of the sieve with a spatula; don't press on the solids or the gelatin will be cloudy. You should have 2 cups of juice. Stir in the sugar until dissolved and let cool.
Transfer 1/4 cup of the raspberry juice to a small bowl. Sprinkle the gelatin over the juice and let stand until the gelatin is evenly moistened.
In a small skillet, warm the gelatin mixture over moderate heat just until the gelatin melts. Stir the mixture into the remaining raspberry juice in the medium bowl and pour it into 6 wineglasses. Refrigerate until barely set, about 1 hour. Gently stir in the fruit and refrigerate until firm, about 1 hour longer. Garnish with whipped cream.
Other fruits could be substituted for the nectarines, but stay away from pineapple, figs and papayas, all of which have an enzyme that prevents gelatin from setting.