Combine chocolate morsels and water in a heavy saucepan; cook over low heat, stirring constantly, until chocolate melts.
Remove chocolate from heat, stir in rum.
Beat egg yolks.
Add cooled chocolate mixture, beat until smooth.
Beat egg whites (at room temperature) until stiff peaks form, fold into chocolate mixture.
Spoon into 8 stemmed glasses or serving dishes; chill 3 to 4 hours.
Beat whipping cream until foamy; gradually add powdered sugar, beating until soft peaks form.
Add a dollop to each serving.