I may have misread the question based on previous similar questions where somebody answering a phone in the U. S. stated that insurance voided for the ferry trip and all subsequent travel on British islands for a car rented on the mainland of Britain. This is pure hokum. In addition to taking cars onto all of the smaller islands, I routinely take cars from the UK to the continent and vise versa. Insurance remains in effect and rental agencies do not prohibit the practice. (This is not the case for movement across the Irish Sea in either direction. ) You have to look at a question posed to AE as though it were asked to a travel forum -- if they don't know the answer they'll give you a guess so they'll appear clever. All they are is a bunch of car brokers. The factual answers have to come from the car owner.
Starting over:
Ferries operate under maritime law which makes the carrier liable for all cargo in the case of loss of the vessel.
Bumps and dents on the vessel are covered by normal automotive insurance just as they are on a land roadway.
The ferry company makes no checks and does not care if the car is owned or rented.
Questions not asked: You need to be lined up at the Ullapool and Stornoway ends an hour before departure and you need to have reservations -- those suckers get full. It's been too long since I've gotten to the Outer Hebs from the Skye end and I don't know what the deal is down there.
Comment: The vehicle lanes on the CalMac boats are wider than the roads on Lewis and Harris and there's a director dude helping you bumper up to the car in front, so there's no danger of hitting anything.