Break up chocolate in a microwave-safe bowl. Pour water over chocolate and microwave at 50% power for 2 minutes. Remove and stir until smooth. Let cool to room temperature.
Place heavy cream into a mixing bowl and mix with an electric stand mixer's whisk attachment until stiff peaks form. Fold chocolate mixture into cream using a rubber spatula until mixture is uniform. Place immediately into a serving dish.
Chill for at least 1 hour before serving. Mousse can be spooned or sliced.