I've been told by Kemwell car rental that I can not take a car on the ferry they said it was an Italian law .
I don't know about Italian law, but we've rented cars in Germany, France and the Netherlands so far. All were from Europcar via AutoEurope and Gemut.com. All contracts said that we couldn't take them on any kind of a ferry. For more information, go to Gemut.com. If you don't find anything on the website, contact Andy. He's the expert on European car rental.
Be careful, and check thoroughly. Consolidators are sometimes full of crap. Lo brings up a statement that's often repeated here regarding Autoeurope. They say you can't take a car on a ferry. I've seen it often. I often go to the far beyonds of Scotland which means taking cars on ferries. Last trip AE had the best rate, so I took a deep breath and went with them. After I made the reservation I called and asked about the ferry business and got the stock answer. I called back and talked to a supervisor and got the stock answer again. Since I'd done it with any number of agencies in the past I knew they were full of crap, so I called back and talked to a second supervisor - - and got the stock answer. I pressed on, marched up to the Europcar counter in Edinburgh (which was who they'd assigned me to) got the car and asked them. They'd never heard of such a nutty notion and I could take the car anywhere I wished. Also, there can be a drawback with consolidators. You pay them up front. They foist you off on somebody. If that somebody can't deliver upon your arrival, you can't walk over to the next counter and ask them to math the deal, which they will. Further, if you have a problem on the road ,it's resolved through the agency, not the consolidator. Last thing: You'll get the best answer from a person at the agency in Italy. People in the US are generally dopes. The list of proving personal anecdotes is long.
More thoughts: a. Here's one guy that had a mess with the same thing: http://www.ricksteves.com/graffiti/helpline/index.cfm/rurl/topic/95015/problems-with-budget.html b. How could the ferry operate economically if it were against the law to transport cars with Italian registration? c. Through the years there have been numerous posts on taking cars across the Irish Sea, most maintained that it could be done. I've always split the rentals because I thought it couldn't. Once as the discussion was going on, I called an agency in London and asked the question - - I was correct. I've driven in Croatia and in Italy. I've never used a ferry between the two. Make sure you're satisfied with your final answer.