Combine vinegar and sugar in a small saucepan over medium-high heat; bring to a boil. Cook until reduced to 1/4 cup (about 7 minutes). Place in a large bowl; cool slightly.
While vinegar mixture cooks, heat a large nonstick skillet over medium-high heat. Sprinkle fish with 1/4 teaspoon salt. Coat pan with cooking spray. Add fish; cook 4 minutes on each side or until fish flakes easily when tested with a fork.
Add 1/2 teaspoon salt, watercress, and pepper to vinegar mixture; toss to coat. Place about 1 1/2 cups watercress mixture on each of 4 plates; top each serving with 1 fillet.