In a small saucepan, sprinkle gelatin over water and let stand about one minute to soften. Heat gelatin over low heat until dissolved. Remove pan from heat.
In a large saucepan over moderately high heat, bring cream, half-and-half, and sugar just to a boil, stirring constantly. Remove pan from heat and stir in gelatin and vanilla. Divide cream mixture among eight half-cup ramekins and cool to room temperature.
Refrigerate, covered, at least four hours or overnight.
Panna cotta may be served in ramekin. To serve on a plate, dip each ramekin into a bowl of hot water for three seconds, taking care not to get cream wet, then run a thin knife around inner edge of each ramekin and invert panna cotta onto small plate. Serve with berries.