In a 4- or 5-quart slow cooker, combine the diced tomatoes, tomato paste, chopped onions, celery, and minced garlic. Pour in the clam juice and white wine (or vegetable broth), followed by the olive oil and red wine vinegar. Sprinkle in the Italian seasoning, sugar, and add the bay leaf. Stir well to blend all the ingredients into a rich, aromatic base.

Cover the slow cooker and let the mixture cook on the low setting for about 4 to 5 hours. During this time, the vegetables will soften, and the flavors will meld into a savory tomato-infused broth.

After the base has developed its depth of flavor, carefully add the haddock pieces, shrimp, chopped clams with their juices, and the drained crabmeat. Stir gently to ensure the seafood is evenly distributed without breaking the delicate fish.

Cover again and cook on low for an additional 20 to 30 minutes, or until the shrimp have turned pink and the fish flakes easily with a fork. This slow, gentle cooking ensures the seafood remains tender and moist.

Discard the bay leaf before serving, and stir in the freshly minced parsley for a burst of color and freshness. Serve hot with crusty bread or a side of garlic toast to soak up the flavorful broth. This hearty seafood stew is a comforting blend of ocean flavors and rustic tomato richness.