Place potatoes in a medium pot; cover with 2 inches water. Season water generously with salt. Bring to a boil, then reduce to a simmer. Cook potatoes until tender, 10 to 12 minutes. Drain. When just cool enough to handle, cut in half lengthwise.
Meanwhile, cook bacon over medium heat until fat is rendered and bacon is crisp, 10 to 12 minutes. Transfer to a paper-towel-lined plate to drain. Break into bite-size pieces.
Transfer potatoes to a large bowl; gently toss with oil and vinegar. Season with salt and pepper; fold in celery and leaves, watercress, and bacon. Serve warm or at room temperature.
I made this last night with the mushroom cheddar frittata on this same site, this potato salad was delicious! We loved the slightly wilted celery and radicchio (I substituted for the watercress as I don't like the taste of it). The smokey bacon was delicious with the combination and the vinaigrette.
I don't often rave about potato salad, but I made this for my husband's birthday with grilled lamb chops and it was a huge hit. A wonderful change from traditional potato salads, it hits a multitude of flavor notes. The slightly peppery watercress and pungent celery drizzled in the sweet-sour white wine vinegar go beautifully with the starchy potato and the salty crispness of the bacon. Looks lovely on a table and wonderful depth of flavor, this is really something special.