Carefully clean scallops of any roe, muscle, or brown connective tissue, which can become tough when marinated. Cut scallops into 1/4-inch horizontal slices. Combine marinade ingredients in a small bowl. Add scallops and toss to coat. Marinate, covered, in refrigerator 5 minutes or as long as 30 minutes any longer and the acid in the lime juice would overcook the seafood.
Meanwhile, prepare salad: whisk together olive oil, lemon juice, salt and pepper in a bowl. Tear watercress into bitesize pieces and toss with dressing. Divide salad among 6 serving plates. Arrange a circle of scallops in the center of each. Thinly slice avocado halves into strips lengthwise and garnish plates along the side. Spoon leftover marinade over scallops and avocado. Serve cold.
c.1997, M.S. Milliken & S. Feniger, all rights reserved