IF you plan on the Argentia ferry book well in advance as you will need a room for the crossing ..

We were thinking of taking the Thursday ferry which leaves at 9 am or the Saturday one which leaves at 8:30 am. Would it still be necessary or advisable to book a room?

If you go from Sydney to Port aux Basque it will take approx. 6-7 hrs. Then you have a full day's (maybe 9-10 hrs?) drive to get to St. John's. The Argentia ferry is 14 hr so I would definitely recommend a berth if you leave from there.

There are plenty of smaller communities between the two coasts plus you will have the opportunity to see the beauty of the west coast and possibly add Gros Morne National Park to your itinerary.

I don't know how far in advance you need to book but you can check availability online. Summer is the busiest time of the year and cabins/berths from Argentia will probably sell out. Also, you are not allowed to stay in your vehicle during the crossing.

If you are planning a day crossing from to Port aux Basques you won't need a cabin, or even a berth unless you are a very bad sailor. There are "lounges", which are large rooms with seating for over 100, a cafeteria, and a bar. You can sit in any of these (you can't, as you were told, stay in your car). For Argentia, which is overnight, accommodation is a must. If your trip begins in mid-August,and ends near the end of the month, you definitely need to book. A lot of people will be travelling around/before Labour Day.

With 10 days, I do think arriving on the west coast and leaving from the east, will make the best use of your time.