Score the whole fish 3 times diagonally on each side. Bring the coconut milk to a boil in a skillet over moderate heat. Add the fish and baste for 3 minutes. Then add the salt, black pepper, sweet green pepper, tomato and celery. Continue to baste for 5 minutes. Simmer the fish in an uncovered skillet over low heat, turning the fish over once, for 10 minutes. Serve warm.