You have to leave the coach to make evacuation easier.
Even in ferries where your coach stays on the top deck, you sometimes have to leave the vehicle.
This is because in case of accident, the fear is that it will take too long to escape the vehicle and the risk of going down with the ferry is high.
And it is not just for rogue waves, it is also for the ferry getting hit by (or hitting) other craft on the water.
Not all short term ferry crossings will make you leave a top side parked vehicle, but those where you are more likely to sleep are more likely to get you out.
As for vehicles parked under decks, they almost always have a 'leave the vehicle' rule with an extra rule of not being allowed on the deck during the crossing.
Anybody sleeping in a car or coach will be unable to escape in case of accident, more so when the ferry capsizes, as the vehicles might move against each other and the 'walls' of the ferry, while those people in cabins will have difficulties but do stand a chance to get out.
Most ferries also keep the crew off those decks, but for a security check at regular (or iregular) intervalls, staying away from the parked vehicles as much as possible.
Keeping passengers from those decks makes for a much smaller chance of thieves breaking open cars parked there, which is an additional reason.
No links, as this is basic knowledge, added to by talking with friends (and chance meeting with crew) who work on ferries, ranging from the local cross shipping canal one to the Inter Island ferry, New Zealand, where I was allowed to accompany one of the staff on a round of the car and train deck.