In large, heavy pan melt the butter. Stir in the flour and cook over medium-low heat, stirring constantly for 20 to 25 minutes or until a dark reddish-brown roux is formed.
Stir in the undrained tomatoes and chilies, water, sliced carrot and celery, chopped onion and green peppers, basil, salt, minimum amount of cayenne (can always be adjusted up later), and black pepper. Bring the mixture to a boil, stirring frequently. Reduce heat and simmer about 25 minutes until vegetables are tender. Adjust the hotness with additional cayenne if desired. (Can be prepared to this point with the vegetables undercooked, then refrigerated to be reheated and completed right before serving).
With the stew simmering, add the shrimp and crabmeat and simmer uncovered for 3 to 5 minutes, until the shrimp are pink and cooked through.
Serve in bowls with stew on the bottom and a mound of rice atop. Serve with a crusty French bread.