If you decide to rent a car at the airport in Athens, you can safely go to Kyllini straight by ferry, which run frequently. Tickets can be purchased directly before sailing in the port. The schedule can be checked here .
The cost of the ferry ticket for a car with a driver and a passenger is about 60 euros.
It does not make much sense to order tickets in advance: once we were taking a ferry before Easter and the Assumption, which are respected days on the island and many Greeks rush to Kefalonia on these days, and that was not a problem to get on a ferry.