yes you need a license because if you get caught by the police you could get a fine or go to jail for the night then get a smaller fine.
A boat can travel between Australia and New Zealand, but it will need to be fully deep ocean capable.
no
yes
Yes
no
No
yes yes
To drive a car in New Zealand you need a driver's license; to get one of those you must be at least 15.
If you are not from either New Zeland or Australia, then yes, you would need an international driving license to drive in those countries.
Given that New Zealand is an island country, the only ways to get there are by air or boat.
By plane or boat.
By boat and by plane