As Tobin said, more robust but slightly less reflective materials like nickle are often preferred for those surfaces. But practically speaking for this application, IIRC you're talking about the difference between 98+% reflectivity and 99+%. In other words, not a terribly significant advantage. You loose more if the mirror is worn, scratched or has some goop or salt on it. You're probably better off with something scratch resistant, which dirt, salt and water doesn't like to stick to, and which doesn't oxidize much, especially around salt, which I guess brings us back to the original question of this thread.