Combine water and turkey in large pot. Bring to the boil. Reduce heat and simmer until turkey is tender, about 1 hour. Using tongs, transfer turkey to a plate and cool, saving cooking liquid. Remove meat from bones.
Heat 2 tablespoons of the oil in heavy frying pan over medium heat. Add okra and cook until no longer sticky, stirring frequently, about 20 minutes; set aside.
Stir flour and remaining oil in heavy large saucepan. Cook over medium heat until deep golden brown, stirring frequently, about 6 minutes. Add 1 litre reserved turkey stock, okra, smoked sausage, tomatoes, peppers, garlic, bay leaf, salt, thyme, oregano, cayenne, paprika and pepper. Cover partially and simmer until thickened, about 1 1/2 hours.
Spoon off any fat from surface of gumbo. Add turkey and prawns; simmer gently until prawns are bright pink, about 15 minutes. (If preparing ahead, cover and refrigerate. Bring to simmer before serving.) Mound rice in shallow bowls if desired. Ladle gumbo over and serve.
If you don't have okra, use frozen peas instead. If using peas, skip step 2 and simply add the peas along with the prawns in the last step.