In a large bowl, combine the soup, milk, and mayonnaise. Stir until smooth. Stir in the crabmeat, shrimp and uncooked noodles.
Spoon into an ungreased 9x12 inch baking dish. Sprinkle top with cheese. Bake, covered, in the preheated oven for 35 minutes. Uncover and sprinkle onion rings over the top. Return to oven and bake for 10 minutes.