Combine gelatin and water in a saucepan. Cook over low heat until gelatin dissolves. Remove from heat. Add mayonnaise, beating well. Chill until consistency of unbeaten egg white.
Stir in next 4 ingredients. Fold in whipped cream. Pour into a lightly oiled 4-cup mold; chill until firm.
Unmold mousse onto a serving plate. Garnish with deviled eggs, watercress, and parsley.