Mix sugar, cornstarch, salt, and egg yolks together in a saucepan. Combine milk and water. Stir into sugar mix until smooth. Cook over low heat, stirring till thick, about 10 minutes. Remove from heat. Stir in vanilla. Let custard cool. Arrange bananas in pie shell. Cover with custard, cover, and let cool. Dress with a meringue or whipped topping before serving.