Place the salmon in a large skillet and season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Add 1/4 cup of the rice vinegar and water to reach halfway up the salmon. Cover and simmer over medium heat until the 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.
Meanwhile, in a small bowl, combine the sour cream, horseradish, the remaining 1 tablespoon of the vinegar, 1/2 teaspoon salt, and 1/4 teaspoon pepper.
In individual bowls, combine the watercress, salmon, and beets. Drizzle with the dressing and sprinkle with the dill.
Technique: Beets are delicious raw, but they should be sliced very thinly. The easiest way is with a Japanese mandoline. You can also do it with a vegetable peeler, or try grating them on a box grater.