Put the cranberries in a small saucepan; add the honey and 1/3 cup water. Cook over medium heat, stirring occasionally, until the cranberries pop and the liquid thickens, about 8 minutes. Remove from the heat and let cool completely.
Toast the walnuts in a small dry skillet over medium heat, about 5 minutes. Transfer to a cutting board and let cool slightly, then roughly chop. Stir the olive oil and vinegar into the cooled cranberries. (Add more honey, 1 teaspoon at a time, if the vinaigrette is too tart.) Season with salt and pepper.
Toss the lettuce and watercress in a serving bowl; season with salt and pepper. Spoon the vinaigrette over the lettuce and top with the walnuts and herbs. Toss before serving.