Place salmon in a skillet and season with ½ teaspoon salt and ¼ teaspoon pepper. Add ¼ cup vinegar and enough water to half cover the fish. Cover and simmer over medium heat until salmon is opaque and beginning to flake, 12 to 15 minutes. Transfer to a plate and refrigerate until cool, 15 to 20 minutes. Using a fork, flake the salmon into bite-size pieces. In a small bowl, combine sour cream, horseradish, remaining 1 tablespoon vinegar, ½ teaspoon salt, and ¼ teaspoon pepper. In individual bowls, combine watercress, salmon, and beets. Drizzle with the dressing and sprinkle with the dill.