Found it out while testing the crash with instruments. If you gain a little momentum on boat and use the last use of an instument (I couldn't make it happen with an axe) the character will be softlocked on the boat and the boat will travel with the last momentum it had but will speed up if you click anywhere. Characters won't move. Boat and character can take still damage in this state. You can actually see it that the momentum is maintained if you use an instrument without breaking.
Ride a boat (sail doesn't matter). Gain momentum. Break an instrument during the movement.