1. Place fillet in a large skillet, and season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Add 1/4 cup rice vinegar and water to reach halfway up fillet. Simmer, covered, 12–15 minutes or until fish flakes easily when tested with a fork. Transfer to a plate, and refrigerate 20 minutes or until cool. Flake fish into bite-size pieces with a fork.
2. Stir together the sour cream, the horseradish, and remaining 1 tablespoon vinegar, remaining 1/2 teaspoon salt, and remaining 1/4 teaspoon pepper in a medium bowl. Add the watercress, and toss to coat.
3. Divide the watercress evenly among 4 bowls, and top each serving with beets and fish. Sprinkle with chopped dill.