You'll still want to check for any condensation. Let the machine warm up to room temps once it gets from point A to point B to reduce chance of condensation shorting anything. Once the metal and things begin to hit the warm air is when condensation will occur until they've fully reached ambient temps and equalized. That's why if a pc has been sitting out in a shop or garage at chilly temps for awhile and it's brought into the house where it's heated it's safest to give it awhile to adjust to the new temps.
Depending how big of an air cooler you opt for, just be careful moving it. Larger air coolers that weigh 700-800g or more if bounced around roughly could potentially flex the motherboard pcb and damage it. That's why when many companies like websites or magazines feature a pc as a prize, they've opted to send the cpu cooler separate and let the user put it on at home. Many with something like the phanteks dual tower or noctua nh-d14 pre-installed were arriving with damage to the motherboard from shipping.
Usually closed loop coolers are the solution of choice for frequent moving such as lan parties and such for the reason that they apply hardly any stress to the motherboard. You'd mentioned the colder region and boat trips, I don't know how long a ferry ride is for you or how long the system might be exposed to unheated storage. For instance if the pc gets packed in a box or crate and loaded separately from the passenger area, like cargo. It's a bit of a special circumstance that doesn't come up often.
You did mention around 8 week intervals though, so leaving the pc in a room for several hours wouldn't be too much of a burden to allow it to warm back up before powering on. Water cooling may not be a completely terrible idea, although I wonder if maybe an actual reservoir, pump, radiator component setup wouldn't be beneficial. That way it could be drained prior to travel (avoid the freezing issue) and refilled once you reached your destination.