Bring 3 1/2 cups water and 1/4 teaspoon salt to a boil in a saucepan. Gradually add polenta, stirring constantly with a whisk. Reduce heat to low; cook 12 minutes, stirring occasionally.
Combine tomatoes, onion, 1 tablespoons oil, vinegar, and capers.
Heat a large nonstick skillet over medium-high heat. Add 1 tablespoon oil; swirl or broth to coat. Sprinkle fish with 1/4 teaspoon salt and pepper. Cook, skin side down, 7 minutes; turn over, and cook 1 minute. Remove from pan. Add tomato mixture to pan, reduce heat to low, and cook 2 minutes. Spoon 3/4 cup polenta into each of 4 shallow bowls. Top each serving with 1 fillet and 1/2 cup tomato mixture. Sprinkle with parsley.