I've rarely been on a boat\ferry to know to ask. Years ago, I've been a walking passenger on a Seattle ferry to Bainbridge Island but never actually driven a car inside the parking spot in any size ferry. How safe is it for the car to park there while the ferry is moving (like in Texas Gulf coast)and even on longer trips that last hours to Canada's New Foundland? Do the cars get scratched, bumped or is the car tied down by chains on the parking spot?

 

I've taken numerous ferry trips on BC Ferries both large and small and never had an issue with my car(s) getting scratched or bumped. The cars are not tied down, simply left in Park (or in gear if a manual) with the parking brake applied. The "lead car" and the "last car" in each line may have it's front (or back) wheels chocked.

On some of the larger ferries, you can request that an antique or "special" collector vehicle be allowed to park on the truck deck as the aisles are wider but only the larger ferries have a separate deck.

On the larger ferries they typically won't let you stay on the vehicle deck as they're closed and the safety concern is with fumes. On the smaller ones with open decks you often simply stay in the vehicle.

 

Can’t speak for American ferries, but I’ve used the UK-France ferries many times and it’s always been absolutely fine. Occasionally if the sea is rough they may lash the cars to the deck, but they use wheel friendly equipment, again I can’t speak for USA versions.