Cape Anne Chowder

2 cups thinly sliced onions
2 tablespoons margarine
2 tablespoons flour
2 cups water
2 cups diced potatoes
1 cup sliced celery
1 1/2 teaspoons salt
1/4 teaspoon pepper
1 pound cod or haddock, skinned, boned, cooked and cut into pieces
1 7.5 ounce can crab meat, drained, flaked
2 cups milk
2 1/2 cups shredded sharp cheddar cheese

Cook onions in margarine until tender; blend in flour. Stir in water, potatoes, celery and seasonings. Cover and simmer 20 minutes or until potatoes are tender. Add remaining ingredients; stir until cheese is melted. Serves 6 to 8.