I can't answer your CDW question, as I always use my credit card for complete insurance coverage. However, there are nine sailings per day from Ardrossan to Brodick on Tuesdays. If you need to be on Arran by a specific time, then it would be in your best interest to reserve your ferry space ahead of time. You can make the reservations from the U.S. by internet or telephone, or you can telephone once you get to Scotland if you've decided on a specific time by then.
If you get to the ferry slip without a reservation, you can go on standby. You'd have a really good chance of boarding the next ferry, as they use one of the larger ferries on that route.
Are you returning to Ardrossan, or are you taking the Lochranza to Claonaig ferry to the Kintyre Peninsula? If it's the latter, you can't make an advance reservation for that one. You just show up at Lochranza and wait in line. That ferry runs frequently as well. Just make sure that you don't get in the queue for the last ferry of the day. If you can't get on, you're pretty much stuck in Lochranza for the night. It's a pretty village, with an excellent castle (free admission), but there's not a whole lot to do.