As Bryan says modern ferries have the problem of getting rid of water off the car deck, all the ferries my company have, have got freeing ports all the way down the car deck so there is not a stability problem, the system in pics you'll be supprised does not use that much water.
As Bob says on the boat he worked on they had drills once a week its the same with calmac ferries, most of our boat have got open car decks at the the stern which is ideal for DGs, you can fight the fire from deck level or from the upper deck looking over the car deck.
David