Once upon a time, when Magic's designers played much faster and looser with creature types, the oceans of the planes of the Multiverse fairly teemed with Ships. After all, who doesn't like a good sailing story? Ships were a convenient blue creature type that fit about any size of power and toughness and could fill the niche of islandwalkers/islandhomers when Serpents got tiresome. Also, they could easily be put alongside Pirates, and those guys are really cool.
But then some flavor considerations came down hard on Ships: Not only were they weren't really creatures - a la Walls, which disappeared and then made a comeback - they also had the problem that stopped Merfolk from appearing for a while, namely that Magic's battles are supposed to more or less take place on land, where Ships can't go. So there hasn't been a new Ship creature since Invasion block, and they've all been retconned to reflect the creature types of their crews - Humans and Pirates and all that.
But even noted hater of inanimate creature types Mark Rosewater was forced to make a concession when zeroephyrt asked him on his Tumblr blog, "Can you please bring back the Ship creature type?"
The reason we got rid of ship as a creature type is because, well, it’s not actually a creature type. The was the same reason we got rid of wall and that’s back, so I guess there’s always a chance.
The reaction from a certain segment of the flavor-geek world was delighted. In the notes of the Tumblr post, shardlab said "You could make Ships artifact creatures with low power and high toughness that give other creatures evasion," while dragontiers wrote "But what about if ships WERE creatures, ala Knight Rider or Moira (Farscape)? That would be pretty sweet." If the "oceanic world" that's on the settings not-so-shortlist ever gets borne out, Ships will have to be represented somehow - perhaps as creatures with a new mechanical consideration that finally allows for the age-old concept of riding or vehicles in Magic.