In a bowl, toss together the scrod, olives, tomatoes, scallions, chiles, lime juice, vinegar, oil, coriander, and salt and pepper and chill the filling, covered, for at least 1 hour or overnight.
Pour enough oil into a straight-sided skillet to reach about 1/2-inch up the sides. Heat the oil over moderately high heat until it is hot but not smoking. Fry the tortillas, 1 at a time, folding them almost in half with tongs to form taco shells until crisp, and transfer them to a paper towel-lined plate to drain. Divide the filling among the tacos, top with the watercress and red onion slices and serve immediately.