Peel alternating strips from the cucumber, leaving some peel to create stripes, and slice into thin rounds. In a large bowl, combine the vinegar, sugar and some salt and pepper. Whisk in the olive oil, then 2 tablespoons of the dill. Add the cucumbers and toss.
Combine the creme fraiche, horseradish, mustard, lemon juice and the remaining 2 tablespoons dill in a small bowl. Season with salt and pepper.
Using a slotted spoon, scoop the cucumbers from the vinaigrette and mound on a large platter. Dip the leaf ends of the watercress in the remaining vinaigrette and mound on the platter. Toss the tomatoes in the vinaigrette, remove with a slotted spoon and mound on the platter. Lay the salmon over the mounds. Spoon the creme fraiche dressing over the top.
Garnish the platter with lemon wedges and serve immediately.