I was just over on the island all last week, there aint no road that runs along the seaside, look at the attached map. There is a small section between Penishaw and American river but most(90%) of that strech you can't see s*** just trees. Don't get me wrong, it's an awesome place but you have to be prepared to hit the dirt and possibly some natives to get to the good bits. Plus the ferry ride costs around $300 for 2 people and a car return, no BP Ultimate and it costs 30c/L more for regular unleaded.
It was pretty rough when we went over but no cars moved, just rocked on their suspension, if anything i'd be worried about salt sprey getting everywhere.
It's good fishing tho- fishies
kangarooislandmap.pdf