A car is a must if you want to see around Waiheke - although there is a good bus system and taxis as well as tourist operators, nothing beats having your own vehicle and travelling at your own pace.
If you choose to take your own car - try the Subritzky Car Ferry - we caught the SeaCat ferry from Bucklands Beach and were there in no time. Its a great trip over and you get a great view of some of the Hauraki Gulf's other islands on the way over.
To get your car over to the island, you need a ferry! This ferry starts at Half Moon Bay, not from Downtown Auckland. It goes every hour with the first beginning at around 6am, and going until 8pm.
You must purchase your ticket 30 mins b4 departure though.
I have not used the Car Ferry to get to Waiheke but Sealink offers a service for $113 (car)plus $26.50 for one Adult / $14.50 for one Child. These are all Return fares. Family and Monthly passes are also available. I recommend that you take a look at the website below for information on the ferry terminals as they differ from the Fullers ones.
It is worth working out how much you will need a car and consider not taking your own and maybe renting for one day to get to the more remote destinations as the bus system is quite good in the more densly populated areas.
(The company used to be known as Subritzky but was rebranded to Sealink in December 2005.)