1. Preheat grill to medium-high heat.
2. Combine first 5 ingredients; sprinkle with black pepper, salt, and sugar. Drizzle vinegar and oil over bell pepper mixture, and toss.
3. Combine butter, thyme, and garlic, stirring until well blended. Arrange lobster tails, back sides up, on a cutting board. Carefully cut shells lengthwise, cutting to, but not through, the middle of the meat. Open halves. Arrange tails, cut sides down, on a grill rack coated with cooking spray. Grill 2 minutes. Turn tails over; divide butter mixture evenly among lobster tails, spreading evenly over flesh. Grill 2 minutes or until desired degree of doneness.
4. Arrange 1 cup watercress on each of 4 plates; top each serving with 1 lobster tail and about 2 tablespoons relish.