This is a really unique adventure, and one of the few to really capture the "Northern theme". I also really like it for the coldest category, but I think it needs some cold mechanics to drive that home.
The coldist kicked up by these ships must be terrible. What happens if someone falls in these freezing water? Do they get stuck under an ice-shelf (fuck, I'm dead I hear the players saying)? I know the ships are equipped to sail these icy waters, but icebergs must still be really dangerous.
The very fact that my mind is racing with what ifs and ideas for mechanics and what not is a testament to how compelling this scenario is.