Manda,
For a bit more detail on OhLiz's post - I posted this a ways back in a Tictoc thread asking the same thing:
If you are thinking of renting (hiring) a car from an individual, JUST like at home, you would not be covered under the private passenger insurance that most individual owners have covering them. This is the same world wide. There is a stipulation in the insurance agreement that you will not be hiring out your car or the insurance becomes invalid. This is the reason that legal, legitimate car rental companies pay higher rates for their insurance coverage. The car insurance for the person you rented it from would not be covering you as well since you are renting the car... essentially you would have no insurance coverage at all.
and if you think you can get away with saying you "borrowed" a car....
If there is no family/blood relation and you are a tourist visiting the island, there would be virtually no incentive for anyone to let you borrow their car for anything less than an emergency. Considering the circumstances, it would be well within in the right of the insurance company to ask you to prove why there was no payment for use of someone's property. How could you prove that? And please remember that any barter arrangement would count as payment, so saying you brought stuff for them and they let you use the car would mean the car was hired out.
Insurance companies have lawyers working for them whose job is to find legal ways not to pay a fraudulent claim. What would you have on your side to contend this matter in court? And of course you would have to attend the court hearings at your own expense or a warrant for your arrest could be placed on you when you would return to the island.
So in simple terms, driving a car from anyone other than a legitimate rental company puts you and those in the car as well as the other drivers on the road at risk. So.... dont do it....