What a wild 'I'm gonna hurl' ride. That's why sailors have sea legs which is a rolling gait to their step. Watch this video and see why. There most of been folks getting sick all over the place during a ride like that.
From what I know the cars are just parked. I'd think and hope the cars wouldn't ram into one another. How would they secure the cars? Seems there might of been a lot bumper to bumper banging on that ride.
On some cargo ships there are little holes in the deck with hooks inbedded. You take a 3-4" wide strap and secure one end into the hook, throw the strap (ususally about 20' long) over the cargo contanier then hook it to the other side and tighten it. Not unlike a belt. You can do it in various ways, but I dunno if this is done on these ferries.
Anyway, it definitely must've been an experience for the passengers. I researched and found that the ferry (the name escapes me) linped into port with a serious list. I'm sure the crew had alot of...um...you know what... to clean up afterwards. I'm sure alot of meals eaten beforehand came out. The wrong way. If I was on that ship I'd of had my life vest on and my eye on the nearest escape passage.