Melt butter in a large pot over medium heat. Add onion and garlic; cook, stirring occasionally with a wooden spoon, until onion is softened, 5 to 7 minutes.
Add chicken broth, diced tomatoes, wine, water, bay leaves, basil, thyme, oregano, and pepper flakes to the pot; cover the pot. Simmer until tomatoes are softened and broth is flavorful, about 30 minutes.
Fold in cod, shrimp, scallops, clams, and mussels; bring to a boil. Reduce heat to low; simmer, covered, until clams and mussels open, 5 to 7 minutes. Discard any clams or mussels that don't open.
