To take cars on ferries you must make reservations. Though there is bus service on the large and mountainous islands of Andros and Naxos, it is much more convenient to travel by car.
Although islanders tend to acknowledge rules, many roads on the islands are poorly maintained, and tourists sometimes lapse into vacation inattentiveness. Drive with caution, especially at night, when you may well be sharing the roads with motorists returning from an evening of drinking.