Ferries are recommended only for short 1-2 hour routes. For longer trips you are risking it that a windy day will turn the trip to a terrible experience and you should really be flying instead. When the winds gets to 6 Beaufort or higher the puke rates may get very high, under certain conditions.

Greek ferries will run up to 8 Beaufort winds (which is hellish conditions, but rare), but 6 Beaufort is common. At 8 Beaufort winds the puke rate will be 100% and many will beg to die…I traveled once as a child and I almost jumped overboard…