Last summer we did the ferry coming south. The one leg was about 24 hours. We had a lot of fish, etc. We bought a couple of the ice chests from the fish packing places (the ones they ship fish to the lower 48 with) along with some gel packs. When the time came, we packed the freezer contents in those boxes and threw a sleeping bag over them. They were just as frozen 24 hours later. Other stuff we packed with ice in one of those coolers (milk, eggs, etc.) A lot of condiment stuff we just left in the refrigerator and it was fine. On shorter ferry rides, we just turned off the fridge and it stayed cool enough.
Dale