In a large saucepan, combine the beets and enough water to cover. Bring to a boil, then lower the heat to medium, cover and cook until crisp-tender, about 30 minutes. Drain and let cool slightly. Under running water, rub off the peels. Halve the beets lengthwise and thinly slice.
In a medium saucepan, bring 1/2 cup water, 1/2 cup vinegar, the sugar, peppercorns, orange peel and 2 teaspoons salt to a simmer, stirring, until the sugar has dissolved. Add the beets, remove from the heat, and let stand for 30 minutes; drain.
Meanwhile, in a small bowl, combine the orange juice, lemon juice and remaining 1 1/2 teaspoons vinegar; season with salt. Whisk in the olive oil. Toss the watercress with half of the vinaigrette.
Arrange the salmon and beets on 6 plates. Top with the watercress and drizzle with the remaining vinaigrette.