From the linked Wikipedia article: "For reasons of naval tradition, submarines are usually referred to as 'boats' rather than 'ships', regardless of their size and shape."

Here's a more verbose explanation which comes down to the same thing: http://www.subvetpaul.com/WhyBoats.htm

During WWII there was somewhat surprizigly unified ship classification and there was hardly any civillian submarines at the time. It was based on tonnage, speed and main gun calibre. Anything smaller than 1000 tons was a boat. Interwar German submarines we're around 500 - 1000 tons, so they we're called "u-boot".

Since WWII names stuck by role of ship. Now we have all kinds of propaganda pieces floating around and ship classification names don't mean anything anymore. A "frigate" might be anything except oil tanker or CATOBAR carrier.

If you would use WWII classification by top speed and tonnage and apply it to modern vessels, Ohio class submarine would be "Submarine Heavy Cruiser" and Typhoon would be "Submarine Battleship".

"This is a ship-shipping ship, shipping shipping ships" : http://imgur.com/gallery/pB0VXKq

No thats the US navy /marines when they go for a run ashore in portsmouth :-)

The USN being dry - the effect of the UK's stronger beers can have unfortunate consequences.