Place potatoes in a medium saucepan; cover with water. Bring to a boil. Reduce heat; simmer 10 minutes or until tender. Drain. Chill. Cut potatoes into 1/2-inch-thick slices. Sprinkle potatoes and asparagus with 1/8 teaspoon salt.
Combine remaining 1/8 teaspoon salt and next 6 ingredients (through sugar) in a small bowl, stirring well with a whisk.
Arrange watercress on a large platter. Arrange potatoes, asparagus, and crabmeat over watercress. Serve immediately with dressing on the side.