To prepare vinaigrette, combine first 6 ingredients, stirring with a whisk.
To prepare salad, add shrimp to large saucepan of boiling water. Remove from heat, and let stand for 5 minutes or until shrimp are done. Drain and rinse with cold water; drain.
Cut a 1/4-inch slice off top and bottom of oranges and grapefruit. Cut away peel so no white pith remains. Cut fruit crosswise into 1/4-inch-thick slices, and discard any seeds. Cut each grapefruit slice into quarters. Arrange fruit on a serving platter. Drizzle fruit with 2 tablespoons vinaigrette.
Combine shrimp, onion, mint, and 1 tablespoon vinaigrette, tossing gently to coat. Arrange shrimp mixture over fruit.
Combine remaining vinaigrette, watercress, and kumquats, tossing gently to coat. Arrange watercress mixture over shrimp mixture. Serve immediately.