Preheat broiler. Place mackerel, skin side down, in a nonreactive baking dish. Pour lemon juice over fish, and let stand 10 minutes, turning once or twice.
Meanwhile, whisk together vinegar, shallot, mustard, garlic, 1/4 teaspoon salt, and 1/8 teaspoon pepper. Let stand 10 minutes. Add oil, whisking until emulsified. Add tarragon and chives.
Remove mackerel from lemon juice, and pat dry. Score skin 3 times. Season with remaining 1/4 teaspoon salt and 1/8 teaspoon pepper.
Coat a broiler pan with cooking spray. Lay mackerel, skin side up, on pan. Broil until fish is cooked through and skin is crisp and browned, 3 to 4 minutes.
Toss watercress with 1 tablespoon vinaigrette. Serve mackerel over watercress, and drizzle with remaining vinaigrette.