Ships travel over coasts (ocean hexes with at least one adjacent land hex) or over open sea (ocean with only adjacent water hexes).
As long as a ship only travels in coastal regions it will never get caught in a storm and it will never drift away.
If a ship is crossing open sea hexes (this is independent of the start and destination region), it may get caught in a storm and drift away. It is then pushed to the side (random) and then carries out the remaining movements.
If a ship drifts away, it always suffers 2% damage without exception.