[llevrok]

from the UK National Maritime Museum web site:

Definition of a boat

The Historic Ships Committee have designated a vessel below 40 tons and 40 feet in length as a boat. However, submarines and fishing vessels are always known as boats whatever their size.

[19th LA Inf]

submarines and fishing vessels are always known as boats whatever their size.

And aircraft carriers are known as "boats" to Naval aviators, as I recall.