What's something almost no one knows about submarines?

The smell.

When a nuclear submarine is at sea, it spends most of its time submerged. The atmosphere inside the submarine is sealed. Part of the function of the submarine's atmospheric control system is to remove carbon dioxide (CO2), as its buildup can be fatal. To remove carbon dioxide, the submarine uses a chemical called amine . When the amine is cold, it absorbs carbon dioxide, and when it is hot, it releases it. The amine is then recycled through a machine called a CO2 Scrubber, which alternately heats and cools the carbon dioxide and pushes the gas overboard, allowing the crew to breathe.

This system is very effective, but the downside is that the amines impart a rather "unique" odor to the atmosphere. This eventually permeates every part of the submarine's interior, including the crew members' clothing and even their skin.

In addition to the amine odor, the submarine crew was exposed to the smells of cooking, hydraulic oil vapor, diesel exhaust, sanitation tank ventilation, and the odors of numerous sanitation tanks, as well as the odors of people living together in the stuffy space. The interior of the submarine became extremely smelly. The crew members became accustomed to it and, after a while, never noticed it. But others did.

While I was serving on a submarine, I had an excuse to go home and take a vacation in my civilian clothes. I was sitting in my airplane seat next to an older woman, making small talk, and suddenly she asked me, "Did you work on a submarine?" Surprised, I asked, "Yes, how did you know?" She replied, "My husband used to be on a submarine. I'll never forget the smell."

Why is a submarine called a boat and not a ship?

 It is a historical convention that goes back to definitions: ships carry boats. Since early submersibles could be carried by ships, and were often tended by ships called "submarine tenders", they were boats, and the naming convention held.

The earliest modern submarines, dating to about 1912, were much smaller than those used in WWII and later, and were often carried by ships to a place near their objective because they typically lacked the fuel needed to make long return trips.

The first practical submarines were developed in the 19th century as small vessels for 1-4 crew that were typically transported aboard larger vessels and deployed from them. Even when submarines became large enough to be considered more ship than boat, there remained a self-deprecating pride among the crews to continue using the diminutive term. The German term for a submarine is unterseeboot which became anglicized to U-Boat, a term that was popularized in WW1 and WW2.

Submarines are very much considered ships however the earliest submarines to enter naval service were formally titled Submarine Torpedo Boats. Submarine Torpedo Boat was rapidly shortened to Submarine officially but the monicker "Boat" stuck as an informal type name and submarines have been known as boats ever since. The Navy's official designation is Ship, Submersible, Nuclear for fast attack subs, and Ship, Submersible, Nuclear, Ballistic for the ballistic missile launching subs.

By the time submarines had grown into ships, the tradition of calling them "boats" had been established and has remained until today, at least to some extent. Make no mistake, a vessel that is 377 feet long and displaces 8700 tons is indeed a ship, regardless of tradition.