|Spectacular view of northern Svalbard*
Here was my problem: I do prefer that PC's and NPC's follow the same set of rules. If there's a chance that NPC ships sink on the way to the island, there needs to be a chance for the PC ships to sink as well - otherwise I'm treating players like the special unique snowflakes in a world built just for them to exploit. It minimizes player achievement; "winning on the easy setting" is unsatisfying. So I needed a way to put some risk back into the journey.
My own way of reconciling these two competing positions is like this: new groups will *always* begin adventuring right on the island of Thule, at the Viking base camp (Trade Town). However, during campaign play, once they return home at the end of the adventuring season, all bets are off. When/if they plan a return trip the following year, they'll need to account for the same risks as everyone else voyaging to the island.
When the group plans a return trip the following year, there will be choices to make to mitigate the dangers of the journey. They could sign on with an experienced captain (and offer back shares of their loot), or buy their own ship and hire their own captain and navigator. They can make riskier choices, leaving early in the season to get a jump on the competition, risking more dangerous sea ice, or play it safe and wait for warmer weather. The amount of cargo will be a factor as well. I'm sure resourceful players would come up with ways to use the spell lists to reduce risks of weather or ship damage. If you're going to have an element of "courageous seagoing exploration" in the game, you owe it to the genre to give the players a chance to make both bold and safe choices.
A number of comments involved switching the discussion from "make it safe to the island/ or die" to "make it safe to the island/ or deal with complications/ or die" - in other words, stop thinking about sinking as an either/or proposition, and add some degrees of failure to the mix - maybe supplies are lost, the ship arrives but is damaged, resources and hit points are gone, that kind of stuff. Great ideas, and it's given me a lot to develop.
Campaign play for the Black City is something I'm putting in the appendix. After these discussions, I'll be adding a section on navigational hazards and complications to cover rules for making it back and forth to the island safely when it's used in an ongoing campaign - thanks for the help!
*The picture is another spectacular view of Svalbard, this one in the northern part of the archipelago in high summer - the photographer's full set is here, creative commons: Kenyai's Svalbard photos.