Preheat oven to 350°.
Lightly spoon flour into a dry measuring cup; level with a knife. Place flour in a 9-inch pie plate; bake at 350° for 45 minutes or until lightly browned, stirring frequently. Cool on a wire rack.
Heat oil in a large Dutch oven over medium-high heat. Add onion; sauté 4 minutes. Stir in browned flour; cook 1 minute, stirring constantly. Gradually stir in water and next 8 ingredients (water through tomatoes); bring to a boil. Reduce heat; simmer 1 hour.
Stir in crawfish, crabmeat, and hot sauce. Bring to a boil; reduce heat, and simmer 25 minutes. Serve gumbo with rice; sprinkle with parsley, if desired.