Preheat oven to 375 degrees. Place beets in a baking pan; drizzle with 1/2 tablespoon oil, and sprinkle with a pinch of salt. Toss to combine. Pour the water into pan, and cover tightly with foil. Cook until beets are easily pierced with the tip of a paring knife, about 45 minutes.
Remove baking pan from oven; take off foil, and let cool completely. Peel beets, and slice into thin rounds using a mandoline or sharp knife. Divide beets among four salad plates, arranging the slices in a circular pattern.
Peel blood oranges: Using a sharp paring knife, slice off both ends. Carefully slice downward, following the curve of the fruit to remove rind and pith. Working over a bowl to catch the juices, cut between membranes to remove whole segments, reserving juices.
Make the dressing: Measure out 2 tablespoons blood-orange juice, and pour into a small bowl. Add shallot, vinegar, pepper, and remaining pinch of salt. Whisk in remaining 1 1/2 tablespoons oil.
Divide watercress and blood-orange segments among plates of beets, mounding them in the center. Drizzle all with dressing, and serve immediately.