In a large pot, bring water to a boil. Boil shrimp for 10 minutes. Let stand 3 minutes. Remove shrimp from stock with a strainer. Peel shrimp and discard the shells. Add onions and bell pepper to stock. Bring to a boil. Add roux, stirring constantly until melted. Let boil for 30 minutes. Add shrimp, crab meat, oysters, and seasonings. Let simmer for 5 minutes more. Turn the heat off and let the gumbo rest for 20-30 minutes. Serve over white rice. This dish serves about 8 to 10 people.