This idea would be perfect for making trade routes or expanding easier, it would be awesome for multiplayer servers, you would have to dredge the sea floor though and make routes that are safe, because you wouldn't want to run your ship too shallow, it would also create the possibility to have ship wrecks created by players, adding a cool touch to the oceans seascape! I think they would have to have pretty cheesy really responsive steering to make them somewhat manageable to navigate
this would be awesome, all it needs are deeper, bigger, oceans. also it needs a mob, preferably one that can destroy ships, sea,serpent or kracken? other than that, this is a truly epic idea, one i hope gets added to vannilla minecraft. diamond for you!!