Learn English – Why are ferries associated with “boat” and never “ship”

TL;DR: The characterization is based on history and tradition.

There are many, many (some confusing) expressed differences between ship and boat, one of the pithier ones being

When a ship sinks you get in a boat, when a boat sinks you get in the water.

Submarines and ferries, as you noted, are boats. That would fit the above definition (most ferries don't have lifeboats) conveniently, but there has to be more. Some yachts, ferries, tug boats, fishing boats, police boats, etc. do carry small lifeboats or dinghies, but they usually don’t graduate to ship status because of that. (A tragic example being the Estonia [515.16 ft, 15,566 gross register tonnage]), a cruise ferry, which sank in the Baltic Sea. It is one of the worst maritime disasters of the 20th century, costing 852 lives. Anyone looking at it would not instinctively call it a boat. But it was built for coastal waterways, not deep, high seas. The coastal waterways puts it into the category of 'boat'. That it was used in high seas didn't make it high-seas-worthy, as history points out. It was not constructed^[1] to be stable on taking on any water.)

Per Mental Floss

Another factor the Naval Institute considers is the vessel’s crew, command, and use. If it has a permanent crew with a commanding officer, it’s usually a ship. If it’s only crewed when actually in use and has no official CO, then you’re probably dealing with a boat. Ships are also usually intended and designed for deep-water use and are able to operate independently for long periods of time. Boats, meanwhile, lack the fuel and cargo capacity for extended, unassisted operation.

The obvious exception to deep-water definition are the commercial fishing boats that go out for months in deep water. However, they don't have a fixed crew.

Marine Insight lists seven differences, all of which must be taken into account. They are

1. Size: The most important aspect that is considered while stating the difference between a ship and a boat is the size... “A ship can carry a boat, but a boat cannot carry a ship.” A mode of water transport that weighs at least 500 tonnes or above is categorised as a ship. In comparison, boats are stipulated to be quite compact in their structural size and displacement. (Submarines were once small enough to be carried on ships.)

2. Operational Areas: Ships are vessels that are operated in oceanic areas and high seas. They are mainly built for cargo/passenger transportation across oceans. Boats are operable in smaller/restricted water areas and include ferrying and towing vessels... Boats are mainly used for smaller purposes and mainly ply in areas near to the coast.

3. Navigation and Technology: Technologically, boats are simple vessels with less complicated equipment, systems and operational maintenance requirements. ...[Ships] are manned using advanced engineering, heavy machinery, and navigational systems.

4. Crew: Ships... are operated by professionally trained navigators and engineers. A ship requires a captain to operate the ship and guide the crew....[T]he size of the crew on a boat depends on the size of the boat. It can be one person or a full-fledged crew depending on the size and purpose of the boat.

5. Cargo Capacity: A boat is a small to mid-sized vessel, which has much lesser cargo carrying capability as compared to a ship.

6. Construction and Design: [Ships] are complicated structures having a variety of machinery systems and designing aspects for safety and stability of the ship. A boat is much simple in construction and build, and has lesser machines and design complexities.

7. Propulsion: A boat can be powered by sails, motor, or human force, whereas a ship has dedicated engines to propel them.

Finally, the most reasonable explanation is provided by The Shipping Law Blog which states:

Specifically, boats are small to medium-sized vessels with hulls, powered by sails, engines, or human force. Some types of vessel are always categorised as boats, regardless of their size or complexity. Their 'boat' status was designated when these types of vessel were small and has stuck despite their future growth.

So, in the end, it comes down to tradition, as so many things do.

_{[1] Causes of the disaster}