Using the ferry system

Has anyone had experience using the cargo ferry system that runs between islands in Vanuatu? Would it be safe for a woman traveling alone?

I travelled on cargo/passenger ships a lot.
They are perfectly safe but EXTREMELY basic, meaning you may have to sleep on a dirty, open deck with no cover to protect you from rain and sun.
And they are not that cheap either, costing 50-70% of air-fares.

I am a woman, and travel all over Solomon Islands (which is culturally very similar to Vanuatu) by similar ferries. As Laszlo says, they are basic, but the local people will look after you. (just don't expect to find anywhere to wash properly, or a decent toilet). My main advice is to put your valuables somewhere safe - I have only lost things twice on boats in 16 years if travelling all over Solomon Islands - once by a pickpocket at the height of the tension, on a very crowded boat, and the other time, someone must have got inside a zippered bag, taking a mobile phone which wouldn't work in SI, and toiletry items, including my toothbrush (It was a big nuisance spending 10 days in an isolated village without one).
Since then, I always sleep on top of my passport, camera and money.