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.