I have found the following site:
http://www.directferries.co.uk/routes.htm which has a nice graphic allowing you to see where ferries go from and to in Europe. It doesn’t look as though Slaerno – Olbia is a possibility. The best alternative seems to be Naples – Palau in Sardinia and then a short trip south to Olbia and then on to Genoa. But… oops. The Naples – Palau ferry no longer operates. Naples – Golfo Aranchi does however! Golfo Aranchi being actually nearer to Olbia than Palau, so not too bad. But hang on, that ferry only runs once a week! What am I doing? Leave this detail until nearer the time! Worth remembering the website however.