Heat olive oil in a Dutch oven over medium-high heat.

Add chopped red pepper and sliced red onion, cook until tender, stirring occasionally.

Add crushed garlic and red pepper flakes, cook for 1 minute, stirring.

Add white wine and boil for 2 minutes.

Stir in diced tomatoes, salt, and water; bring to a boil.

Add cod, mussels, and shrimp; bring to a boil.

Reduce heat to medium-low, cover, and simmer until cod and shrimp are opaque and mussels open.

Remove from heat and sprinkle with basil.