It all depends on your definition of the word "ship."
"Officially," from dictionary.com, the definition of Ship we are interested in is: Noun, "a vessel, especially a large oceangoing one propelled by sails or engines."
Since the large, oceangoing port is optional, we can look at the definition of Vessel: Noun, "a craft for travelling on water, now usually one larger than an ordinary rowboat."
That's just one definition however. In my military background we were always told a ship was a vessel large enough to carry it's own boat. Considering the size of some vessels I've seen carrying dinghies, that doesn't generate a very large threshold.
A century ago, if you'd asked an old salt what a ship was, he'd probably have told you it was a sailing vessel with three or more full-rigged masts.
So really, there's not much of a point in arguing over what is a fairly loose term to begin with. The site has its standards, and they allow us to post almost any interesting ships, boats, vessels or craft we encounter. Let's stick with that.
Kind Regards,
Kyle