Place the salmon in a large skillet; season with ½ teaspoon salt. Add 2 tablespoons of the vinegar and enough water to reach halfway up the salmon.
Simmer, covered, until the fish is opaque throughout, 10 to 12 minutes. Transfer to a plate and refrigerate until cool, at least 25 minutes. Using a fork, flake the salmon into pieces.
Meanwhile, in a medium bowl, combine the sour cream, horseradish, the remaining tablespoon of vinegar, ½ teaspoon salt, and ¼ teaspoon pepper. Add the cucumber; toss to coat.
Divide the watercress among bowls, top with the cucumber mixture and the salmon, and sprinkle with the dill.