Here's a fact: Heat travels in one direction only; from warm to cold. So if a temperature difference exists between two items, the warmer item will lose heat to the cooler item; so the warmer item gets colder, the colder item gets warmer.
Simply put, when we put warm beers into a cooler of ice, we don't actually cool down the beer as much as warm up the ice! When heat leaves the beer for the ice, the beer gets colder as a result, and soon the contents are all at a wonderfully refreshing 32F temperature. And thanks to the laws of science and beerology, everything will stay that way until all the ice has melted and the beer has been consumed.
And now here's a conundrum: Should you endeavor to keep your fridge and freezer as full as possible in an effort to save power usage?
We'll first have to consider the two configurations of insulated boxes typically found on boats: those with a top opening hatch, and those with a front-opening door.
With a top opening hatch, little to no cold air will be lost when the hatch is opened, but when a front-opening door is opened, cold air will rush out from the bottom to cool your feet, and warmer air will be drawn in from the top to replace it.
The more well-stocked a front-opening box is, the less air there is to lose when the door is opened, and so less refrigeration run-time is required to cool down the warmer air now in the box. It's not as bad as it might seem though, because if you dumped all of the 40F air from an empty 10 cu ft refrigerator and replaced it with 80F air, it would take just 2 to 3 minutes of refrigerator run time to cool the new air. However, if the replacement air contains any significant amount of moisture, it could lead to a heavy build-up of frost on the evaporator plate, and that will significantly affect the efficiency of the refrigeration system. Excessive frost build-up can also be the result of poorly fitted and leaking seals, but in this case it happens when the doors are closed.
For a front-opening fridge and/or freezer, the best solution is to have magnetic seals, like those found on the Vitrifrigo double-door DP 2600. Magnetic seals prevent cold air escaping when the doors are closed, which in turn negates the need for warm, humid air to be sucked in to replace the lost cold air, as happens with poorly-sealing compression gaskets.