This version of the classic Latin dish is lower in fat because the seafood is grilled instead of fried. Any firm-fleshed fish could be used.
Cut red onion in half; dice one half and cut the other into very thin rounds.
Combine 1/2 cup parsley leaves, chopped onion, tomatoes, wine, vinegar, lemon and lime juices, salt, black pepper, cayenne pepper, 1/4 teaspoon red-pepper flakes, and 3 tablespoons capers in a blender. Process until smooth. Strain.
Spray grill with cooking spray; heat to medium-hot. Toss shrimp and scallops with oil and remaining red-pepper flakes. Grill swordfish 3 minutes per side; scallops and shrimp 1 to 2 minutes per side (they will be slightly undercooked).
Meanwhile, arrange half the remaining parsley sprigs, half the orange slices, 1 tablespoon capers, half the garlic, and half the sliced onions in a large shallow dish.
Add grilled fish to dish. Pour tomato mixture over; cover with remaining parsley, oranges, capers, garlic, and onions. Cover with plastic wrap and refrigerate. Marinate overnight. Let stand at room temperature for 30 minutes before serving.