Toss together cabbage and salt in a large bowl, and squeeze to slightly soften leaves. Place a small plate, weighted with a heavy can on top, directly on cabbage to release liquid. Let stand at room temperature 1 hour.
Drain and discard cabbage liquid. Add scallions, carrot, chiles, and vinegar to cabbage; toss to combine. Pour soda over vegetables, cover with plastic wrap, and refrigerate overnight. Fold watercress into kimchi just before serving.
