Melt the butter or margarine in a large saucepan over medium heat. Add the onion then saute for 5 to 10 minutes or until tender. Add the potatoes and water then simmer for 20 more minutes or until potatoes are tender.
Add the creamed corn and milk, stirring until smooth. Finally add the fish, stir well and allow to heat through, about 10 to 15 minutes. Season with salt and pepper to taste.