Subs and fishing vesssels are always called boats. No Idea why except that you dont get many subs under 40 feet or 40 tons. (can of worms opened now - Oh yes you do I can hear them say). Oil rigs are commonly known as platforms as they are towed and fixed. Ie no engines or sails.
The national Historic Ships register criteria to register a vessel is as mentioned on my first post. 40 feet or 40 tonnes. No subs or fishing boats in this category.
Also mentioned in other sites is the fact about a ship can carry a boat but not vice versa.
Another method is she is over 150 feet with 3 or more gaff or square rigged sails. Older shops.