Playing solo, you didn't have to log off. Just hit escape to pause the game.

Jumping overboard will stop the boat in its tracks.. you can log out there.

If you lose connection or something and the boat does travel it can go out of visual range quickly... although you won't see it, it won't be far and if you swim the direction you were sailing you can find it. If in a situation where your internet crashed or whatever and you have to swim far, you can log out and back in while in the water and it replenishes your stamina so you don't drown. I wouldn't use that if I got myself into trouble but if something beyond your control puts you in a bad spot like that it's an easier method over console commands.

Don't forget to bring a towel.

Does the pause function help you while sailing in single player? It's done by pressing the Escape key.

It would be worth to report this on Valheim Bugtracker, in case devs wish to work on the synchronization of character and boat prior to save.

lol