Yes you are right, it is 6 days in a 1 month period, but the problem with the 6 days pass is that it is only a 4 days pass if you are traveling only in the Greek islands:
"4 out of the 6 days can be used on domestic routes within Greece and 2 out of the 6 days can be used on international routes between Italy (Venice, Ancona or Bari) and Greece"
So it would be useless for you.
Even if you have 4 ferry trips on 4 different days during 1 month period, it will still cost you less by booking directly with the ferry company.
For this pass to be profitable, you would have to spend all your holidays on the ferries without ever disembarking from the boat.
As I wonder how it works, I read the details of the reservation process for this offer and I don't understand because they say that you have to go to the ferry company's websites and reserve the places normally. At no time in the booking process is there the possibility of indicating that you have this Pass and not paying the normal ticket price.
Anyway, for reasons of schedule compatibility, impossibility of changing economy seats into cabin seats, and the price difference, I'm sure it's not interesting for you. And if it is, I'm ready to offer you an all-you-can-eat "Greek Salad" package for 1 year :))
