Biggest prep for the trip is the ability to bust out your wallet, and fork over the dough Everytime I have gone over, it's been a gale wind. Always fun. I wouldn't stay in the car in case the ship sinks.
I took the ferry often when I was living in delmarva. I would call ahead and get a reservation just to gaurantee a spot esp if you going on a weekend in the spring summer fall time. It just makes it hassle free to show up and go right on board. if your going during the season the bars will be open, some ships have two or more nicer bars. Otherwise just sleep in your car. And there is at least one pirate you should be aware of at all times. The other thing that sucks about the ferry besides the steep price increase every yr. is the fact that it takes an hr or longer to go like 10 or 13 miles. You think the boat is moving but it is only going like 7-10 knots.