Most ferry companies allow you to pick up your tickets directly at the port of departure at their central agency (they may not be in port).
During the booking process, you will be asked to choose how you will receive your tickets.

To pick up your tickets at the port before the departure of the ship you must, if there is an option, select "Pick up from the port" and the confirmation of your reservation as soon as it is completed will show relevant information.

Please note that in order to pick up you must have with your id/passport and also the system code or the reservation code (which is mentioned on the booking confirmation page in red letters)