There might be icebergs or sandbars or rocks or volcanic activity or wars blocking the sailing route too. They seem like equivalents.
The longest mettled straight route with minimal deviation would also be an interesting exercise.
Extremely highly inprobable vs 100% there and directly impassable unless walking & doing some proper alpinism is hardly equivalent
Extremely highly inprobable vs 100% there and directly impassable unless walking & doing some proper alpinism is hardly equivalent