If you've had your heart set on a road trip around Hawaii, this is your lucky month. That's because after much anticipation, the Hawaii Superferry is set to begin service on August 28. The service will link Oahu with Maui and Kauai, and will hold 866 passengers and 282 cars.
Fares for weekday travel on the Superferry are $44 per adult with 14-day advance purchase, and less for kids and seniors. Cars cost $59 each. How does the ferry price compare to flight costs? On a sample weekday in September, I found fares on Go! For $30 between Honolulu, Oahu, and Lihue, Kauai. On Aloha, there were sale fares from $49, but they were sold out and the cheapest bookable fare I could find came out to $94.
So the ferry is competitive, but not necessarily the cheapest option. However, if you want to bring a car between islands, driving onto the ferry is a whole lot easier than trying to check it as baggage.