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.