I use an adapter too. Plug into mains then plug cooler into attached socket. Not sure about on ferry we don't carry food really but if you are going to France many sites have fridge hire. I'm going tomorrow and have my fridge booked already.
can anyone recommend one?, we're on a budget (£50 max) and so far most i've looked at run off the car. really want a mains powered one, or is the answer to buy an adapter so we can plug it in, is it safe to do that?