Heat oil in nonstick Dutch oven over medium heat. Add bell pepper, scallions, celery, and garlic. Cook, stirring until softened, about 5 minutes.
Add tomatoes, water, wine, and pepper flakes, bring to a boil. Reduce heat and simmer, covered, until flavors are blended, about 10 minutes. Stir in corn
Add shrimp, mussels, and halibut to the pot and return to boil. Reduce heat and simmer, covered, until mussels open and the shrimp are just opaque, about 5 minutes. Discard any mussels that don't open.
Divide stew evenly among 6 large shallow soup bowls.