I see the AI just go to the coast and immediately get on their ships, I wanted to know if this is possible for human players. I haven't played the viking dlc so I don't know much about the ship system.
No, it's not possible to do that atm.
You can only sail major rivers and seas on the ships. If you want to traverse the land, you have to leave the ships in a port of a city or on a landing spot. I read somewhere that leaving them on landing spots is unsafe, but I've never had anything happen personally.
For AI it is simplified, they get a corresponding to their army size boat every time they decide to set sail. That way game can run better, without keeping track of every party in the game.
For additional information, you might want to visit VC wiki. Information on this page applies for this mod too, except for prices and stats of the ships.
http://mountandblade.wikia.com/wiki/Boats