Okay, this is just awesome. I have no idea how long this has been in Skyrim, if it was in the base game, an update or included in DAWNGUARD or Hearthfire. I was in Dawnstar, outside Silus's house, the dude obsessed wih the mythic dawn, and there's a guy sitting on a boat. I talk to him, only to find that it's a ferryboat, like the carriages, and can travel to Soliude or Windhelm.
This is seriously epic, I'm using a roleplay character and I don't use fast travel, so this is a great little tidbit to add depth and immersion to the game <3
So, er, basically, epic.

I belive Dawnguard added that

I like it too (also for RP reasons, although I'm not that strict about no fast-travel rule). It would also be awesome if they added a carriage of boat to every small town and settlement... I mean, Ivarstead for example might not have many inhabitants, but it surely does some business with Riften and Whiterun... and business means that someone needs to go back and forth between towns.

This had to be added by Dawnguard.

yeah, shame it's made useless by fast travel

but i guess fast travel is optional. i would've preferred a network of different travel methods like morrowind had

Confirmed. No such boats in my game