It only takes a small amount of water to get onboard, and it sets up a sort of uncontrollable rocking effect that actually turns the vessel over quite quickly. They can sink in no time flat.
It's a roll-on, roll-off ferry, and there is big question mark over the stability of this kind of ship. It would only take a bit of water to get on board this ship and it would be all over. ... The percentage of this type of ferry involved in this type of disaster is huge.