Special equipment: 4 (4 to 6-ounce) molds, preferably metal
Make the Bloody Mary: Put into a pitcher the lemon and lime wedges, horseradish, celery seed, cilantro, Worcestershire, and hot sauce. Muddle these ingredients by smashing them together with a wooden spoon. Pour in the juice and the vodka; stir, taste, and season with salt and pepper. Serve on the rocks as a beverage or pour into the molds and place in the freezer for 4 hours or overnight.
Poach the shrimp: Put the onion, lemon, bay leaves, and peppercorns into a medium size pot and fill it with water. Bring to a boil, add the shrimp, reduce the heat, and simmer until the shrimp are cooked through, about 5 minutes. Drain and refrigerate. When they are cool enough to handle, peel and devein them.
Serve: Unmold a Frozen Bloody Mary onto a chilled salad plate. Toss the shrimp with the dressing and divide them among the plates. Garnish with watercress and a lemon wedge; drizzle over a few drops of olive oil.