Thanks for the answer, but let me clarify one thing here : My players and me are NOT fighting. Most of the responses here seems to see a fight between players and GM. It's not the case. Everyone is like "your players are stupid, they should follow the Path, why do they play this!!!" They play this because they wanted to play an Adventure Path with a lot of roleplaying, and they wanted me to DM them, but I was very busy (and rusty) so I asked them to use a published adventure. But the players have been frustrated by the Path when they had a 4-5 sessions long dungeon (Really, who the hell designed Brinewall Castle. It's way too long) when they wanted to roleplay. Last game, after the assault on Asvig house (in which the players devised a clever plan that made us skip most of the combat, they asked me to drop the books encounters, follow the story arc but design the encounters myself because I would be better without a badly designed crutch (their words, not me).
Again,my players don't want to take a boat just to be pain in the ass - but because it seems more logical for them to pass through sea then through a frozen inhospitable wasteland. I can't really fault them - they are thinking logically with the money they have. ANd the fact that the price of the viking boat is officially triple what is written in the AP did put me in a very uncomfortable position.