All ferries are now required to have an indicator light on the bridge signalling that the bow / stern door is closed, locked and an interlock put in place. That way, it should be impossible for the ferry to sail with the watertight doors still open. That said, there are a lot of cowboy operations out there.
The bulkheads are only placed in the engine room to stop free surface effect in case of water entering. The main car decks themselves should have scuppers (drains) to allow water to drain freely.
Very sad, especially after all the new regulations have been put in place to stop this type of thing happening. Doubtless the owners will get off scott free.