In a small bowl, whisk together soy sauce, 3 teaspoons honey, and orange juice; season glaze with salt and pepper. In a large bowl, whisk together 1 tablespoon glaze, 1 teaspoon honey, vinegar, and oil; set dressing aside.
Heat broiler, with rack 4 inches from heat. Line a rimmed baking sheet with foil. Season salmon with salt and pepper and broil 5 minutes. Remove fish from oven, brush with glaze, then broil until opaque throughout, about 2 minutes, brushing with glaze once more.
Add orange segments, watercress, and onion to dressing and toss to combine. Season with salt and pepper. Divide salad among four plates and top with salmon. Serve immediately.