Sounds to me like the door and hatch seals are leaking badly John, because once they are all shut and you are away from the boat, there should be no more air getting inside the fridge. What moisture remains in the air within will condense, and that should be it until you next open the fridge door/hatch. So I'd start looking at the seals to make sure that they are touching all the way round, and if not, it could be that one of the doors or hatches is slightly warped, thereby allowing the fridge to breathe and draw in moist air.