I served on the U.S.S. Independence. We called her, Indy, Chicken of the Sea, Flat-top, Ship, and maybe a few other names, but we didn't call her a "boat". I just scanned through my 1961-61 cruise book (The Profile) and all references I found were "ship".
Calling her a "boat" would have been as inappropriate as calling a "piece" a gun. Piece is the naval term for a rifle, for example. A"Gun" is that big old cannon that goes boom on a destroyer or similarly armed vessel. lol