Peel pears, leaving stems in place.
In a 3- to 4-quart pan over high heat, bring cranberry juice, vinegar, and sugar to a boil; add pears. When mixture resumes boiling, reduce heat, cover, and simmer until fruit is tender when pierced, about 8 minutes (12 minutes for Bosc pears); turn fruit over halfway through cooking.
With a slotted spoon, transfer fruit to a rimmed plate. If using Bosc pears, cut each in half lengthwise.
Boil juice mixture over high heat, stirring occasionally, until reduced to 2/3 cup, about 10 minutes.
Meanwhile, cut baguette diagonally into 1/2-inch-wide slices. Brush both sides of each slice lightly with butter. Arrange slices in a single layer on a 14- by 17-inch baking sheet. Bake in a 375° oven until golden, 10 to 12 minutes.
Set each whole pear upright on a plate (lay Bosc halves flat sides down). Spoon syrup equally over or around fruit. Arrange toasted baguette slices, cheese, and watercress next to pears. To eat, cut pieces of pear, dip in sauce, and combine with bites of toast, cheese, and watercress.