The ferry trip from Piraeus, the port of Athens, to Syros is scheduled as about a 3-hour trip. We sailed at 5:30, estimated arrival before 9. We were on the Blue Star Paros, a solid car-carrier ferry, and one we had been aboard before. It's typical of the bigger ferries on the Greek island routes: single stern ramp, conventional bow, capable of carrying semi-trailers on the car deck and a few hundred people on the passenger decks above.
We took up seats outside, on the aft deck, right by the rail.
After about an hour, the sea started kicking up, and the spray was sufficient that we decided to move to different seats on the aft deck, further inboard and protected.
The weather started getting rougher. I am no student of rough waters, but the waves were big, definitely at least 3m tall, and depending on how you measure them, I could easily claim 6m, maybe even 9. The ship started to move in axes I didn't even know were possible (roll is obvious, pitch is not surprising, but many of the wave events were inducing significant, sudden YAW in the ship's heading).
The most severe rolls were severe. I know it's far too easy to overestimate list angles on a ship, but I'm going to straight-facedly claim there is no way we rolled less than 20 degrees in the worst events, and I think it was much more than that. As the ship heeled over, empty chairs on the deck started falling over. Walking was impossible during even moderately severe rolls (compared to the worst), and the ship had taken a modest but consistent list to port.
About 90 minutes in, a passenger on the rear deck vomited, the first one I saw. About that same time, a severe roll toppled another passenger right out of her chair (she wasn't seriously hurt). Then the ship PA requested a doctor. Then we decided to go inside, as TLO was starting to feel not so hot. We had to wait a minute or two for the rolling to subside enough to make a try.
We got 6m inside the door, Rebecca lurched to a stable countertop, and then she couldn't move. The cabin was suffused with the stench of vomit; despite a ready supply of sick bags, not everyone made it. We were stranded at the counter, unable to move further into the cabin (where I thought we could get a chair that was bolted down). After this break, we continued forward on the ship, looking for a safe spot.
We got another 6m down the corridor before TLO was overcome, and we lay down right there in the corridor. There were no safe places ahead to walk to, and she had neither the legs nor the stomach for further activity (all she had, she later confessed, was a burning desire not to become one of the vomiting ones: she stayed true to that).
The crew was very good: multiple people checked on us, offered ice, and what assistance as they could. Finally a group of them came rocketing by with an actively vomiting (into a sick bag, mercifully) passenger in a wheelchair, and promised to return for us. They did, we bundled Rebecca into the chair, and got whisked to the front of the ship, where the first class lounge was doing double duty as a sick bay. Rebecca got a clean sheet on the floor and a pillow, a quick inspection by a doctor, and rode out the remaining 3 or so hours of the cruise there.
It was absolutely the worst conditions I have ever experienced on a ferry. I was genuinely alarmed by the attitude of the vessel several times, and it's no exaggeration to guess that 10% of the passengers were overcome enough to vomit. Chairs fell over, there was apparently glass breakage in the cafeteria cases designed to prevent exactly that, and there was apparently some kind of substantial medical incident aboard (I'm guessing injuries from a fall.)
We made it to Syros fine, at least an hour later than expected. When we finally got into the shelter of the island (and especially inside the port breakwater) everything returned to normal. We were escorted downstairs and to the front of the line, given Rebecca's indisposal, and when we got there we saw the luggage rack had spilled all over the car deck. Fortunately, our bags had been thrown the furthest (at least 3m from the rack) so they were easy to retrieve.
Disembarkation proceeded with no further incident.
And then it took 30 minutes for the rental company to find a rental car for us. That didn't seem like a very big problem, though.