This may help to clarify your rights and should be sent with your letter of complaint...
New EU regulations guarantees reimbursement or rerouting
New European Union regulations on passenger rights in "maritime and inland waterway transport" – including ferries – come into force on 18 December 2012.
Under this legislation, passengers will be guaranteed reimbursement or rerouting, if there ferry is cancelled or the departure is delayed by more than 90 minutes. They will also gain "adequate assistance" for snacks, meals, refreshments and, where necessary, accommodation for up to three nights. Financial coverage will be up to €80 per night.
If the arrival of the ferry is delayed or the crossing cancelled, compensation of between 25% and 50% of the ticket price will be awarded.
Terry