Divide the watercress and salmon among 6 plates.
In a large bowl, combine the horseradish, mustard, sour cream, lemon juice, salt, and pepper. Transfer half the dressing to a small bowl and set aside. Add the cucumber to the large bowl and toss. Divide the cucumbers among the plates and sprinkle with the dill. Serve with the reserved dressing, lemon wedges (if using), and bagel chips or baguette slices.