The ships getting stuck during travel hit me during my first playthrough. I was forced to leave Firese after following the storyline to get revenge on Sven and being betrayed by a Jarl. So, logically I went to the nearest port and tried to get out. This prompted me to move a small distance, and get stuck. Saving & Exiting was my only apparent option and as I use the single save game mode, I essentially got stuck there with no way to backtrack. It is rather disappointing to see this occur in the main narrative path of the game, on my first playthrough. I was worried I would be stuck completely, it certainly looked that way. Finally I did find a way out, and I hope it helps other players:
I have noticed the ships move a short distance forward and then get stuck. By saving, exiting and reloading the game to allow them to move a little again, you can actually get to your destination .
I actually lost track of the ships as the camera would not move, but after a few minutes of saving, exiting, reloading and waiting a few seconds while I assumed they would move a few paces, the ships did actually reach the port of destination and I was able to continue playing. Seemingly normally. I must have saved/loaded over a dozen times to get to Dunwic from Dorestad.
I really hope this works for you guys!