Squeeze juice from pomegranate halves using a citrus reamer or juicer to measure 1 cup. Combine juice, vinegar, soy sauce, 1/8 teaspoon black pepper, and red pepper in a small saucepan; bring to a boil. Reduce heat; simmer until reduced by half (about 15 minutes), stirring frequently. Keep warm.
Rinse scallops; pat dry. Heat oil in a cast-iron skillet over medium-high heat. Sprinkle scallops with salt, sugar, and 1/8 teaspoon black pepper. Add scallops to pan; cook 2 minutes on each side or until done.
Arrange 1/2 cup watercress on each of 6 plates; divide scallops evenly among plates. Drizzle each serving with about 1 tablespoon sauce; sprinkle with 2 tablespoons seeds.