My husband insists that a “ship” is determined only by length, but I think that ferries, like these are not ships, but boats.
I know it’s a silly thing to wonder about, but, well, you know. 
Old Sea Dog: “A boat is something that lifted up and put aboard a ship.”
While there’s no hard and fast definition of size, a ferry would be considered a ship by most mariners.
So Washington State Ferries certainly seems to think so.
One simple (but rather pat) definition is that a boat can be carried by a ship. Of course, in practical use certain types of vessels (submarines in particular) are called “boats” for historical reasons, even though the biggest submarines are comparable in size to vessels that everyone considers “ships”. And there are floating drydocks that can carry some of the biggest “ships”.
IMO, I’d refer to a big ferry like the one you link to as a ship – it’s pretty big, and it’s certainly not carried routinely by any other vessel.
My reasoning is ferries are called ‘ferry boats’. I’ve never heard of a ferry ship, unless Tinkerbell took over Cap’t Hook’s.
And salvage ships, too: http://upload.wikimedia.org/wikipedia/commons/3/32/MV_Blue_Marlin_carrying_USS_Cole.jpg
I was on that very ferry just two weeks ago and I thought of it as a boat. No way did I see it as big enough to qualify as a ship. And Bremerton, one of the docking sights, is a Navy town - I’ll bet the sailors don’t consider them ships.
I’m sure some Navy vet will correct this Marine Corps vet if I am wrong, but as far as the Navy is concerned all submarines and surface vessels 165’ or less in length are considered boats.
Great Lakes freighters are commonly called boats rather than ships, in spite of the existence of a 1000 foot class. There isn’t really a universally agreed upon rule. I’ll consider ferries boats, for the simple reason that I’ve always heard them referred to as “ferry boats”, not “ferry ships”, as mentioned.
I would call it a boat since it is not designed to operate at sea for any extended period.
I would call them boats. They do not require an unlimited licience for the mates or engineers. they are limited in Hp and tonnage.
In the UK we wouldn’t say ‘ferry boat’, just ‘ferry’. This one is pretty much a cruise ship that happens to be able to carry 650 cars along with its 2400 passengers.
Yes, ferry routes within Britain are as much as 14 hours (between Aberdeen and Lerwick). There are even longer routes going from British ports to Spain, Denmark etc. Generally people would refer to those ferries as “ships”.
Surely the point is that “ferry” defines the function of the vessel, not its type, and they can range from thisto this.
One of those is a boat, the other isn’t 
The ferries in the OP are somewhere in-between and open to all salty sea-dogs to argue over on the grounds of size but not because they are ferries.
First off, there’s no Ministry of Language which defines the rules of things. You can call it whatever you like. Most of the time, a boat is a either a very small vessel or a river/lake craft. A ship is a large vessel or a seagoing one. But there’s no formal dividing line except possible public humiliation among those who disagree with your definition and would mock you publicly.
Kind of like the whole nonsense about the season starting at solstices and , apparently because some oddbody put it down on a calender or something. There’s no hard and fast rules, or for that matter soft and slow ones, and I favor concepts which make some sense.
Technically, a dinghy is a watercraft without living quarters. Huge America’s Cup racers are dinghys. Is a ferry a dinghy?
‘Ferry’ is what it does, not what it looks like.