I think they are called she's because
Subs are often referred to as boats.
fiancée
boats do not have a gender. men may say, "shes a beauty, aye?" but that doesnt mean that the boat is a female.
Boats covered in iron are commonly referred to as ironclads.
German submarines were referred to as U-boats during the world wars.
German submarines were referred to as U-boats during the world wars.
A group of boats is usually referred to as a convoy. If they are a medical group of boats it is normally called a Medical Convoy.
A group of kayaks are commonly referred to as a fleet, the same as most boats.
All ships are a she. Titanic was one of three Olympic class ships. Her sister ship Brtannic was sunk during WW1 when being used as a hospital ship. he but it has a sister britannic that fell too.???
Potato skins are generally just referred to as potato skins or occasionally they may be referred to as potato boats.
They were referred to as "Jeep" carriers .
Submarines are commonly referred to as "boats" due to naval tradition, which dates back to the early days of maritime language. Historically, smaller vessels were called boats, while larger ships were referred to as ships. This terminology has persisted, and even though submarines are technically large underwater vessels, they retain the "boat" designation as a nod to their origins and the conventions of naval terminology.