Hi the cost for the ferry is 33 euro if you return within 3 days, you dont pay for the passengers.
It is correct you dont need to book but you can not buy the ticket sitting in the car while you are in the queue as you could do somewhere else. You or your passenger have to run to the ticket counter while the driver carry on stay on the queue with the car! if you dont have a passenger you need to stop the car and run to the ticket counter! I heard that is possible to buy ticket also at petrol stations on the motorway before to get to the ferries but i can not confirmed this.
The ferries run continuosly therefore the waitng time it is normally quite short, at the end of July we waited just half an hour to get on board.
I suggest to take the private ferry, the Caronte & Tourist because the state ferry often have to wait for the train before to leave and if the train is late, the ferry will not leave.
You can view more information and pictures of the ferry and the ticket counter in this page:
and also view the Gallery 2 (inn the same page) for pictures taken at the ferry in July and August 2009
ciao
Davide
studioitalia@manx.net