Boats https://www.usni.org/magazines/naval-history-magazine/2017/october/bluejackets-manual-ships-and-boats-and
Now for some added confusion. Using the above guidance, submarines are technically ships. Yet they are traditionally referred to as boats. The original submarines were very small and manned only when in use, so “boat” was appropriate. But as they developed into larger vessels—and rightfully should have been called ships—the original term stuck. When the large nuclear subs began to appear, there was an attempt by some submariners to start calling them ships, but as with many things in the Navy, tradition trumped logic, and today, all submarines—even the giant “boomers” (fleet ballistic-missile submarines)—are called boats.
