[Wow -- that mast-spike idea is really good! We never came up with an idea like that!]
Most of this has been covered already, but I'm going to post anyway.
My group wasn't Viking-like at the time, but they were always worried about sea monster attacks and had the idea of a removable mast for a vessel on which the mast usually wasn't removable. The idea being that the creature probably tends to grasp the mast, so if we could remove it then that's just one less thing, right?
But after the removable mast was implemented it was decided that it simply takes too long to take the mast down and so the idea was forgotten.
Until the group had to hide their vessel in a cove one day and after dropping anchor <PERSON> said, "Okay; let's get to work pulling out this stupid mast!" The other party members looked at each other in surprise and said, "Oh yeah -- we have a removable mast!" and it ultimately worked to their great advantage.
Anyway, the best defense my group found against cephalopods was to use poisoned spears and harpoons. Where this probably falls down is if the cephalopod is under the magical control of another intelligence or magically bound to a place, or if the game mastering is of poor quality.
I could go on for three or four chapters on how to properly manage monster attacks, but suffice it to say that cephalopods are unlikely to:
a) show more than their tentacles above the water line
b) fight to the death
c) continue to fight for an extended period of time if the opponent has any "bite" to it. Cephalopods are incredibly intelligent -- for animals. They can be very dexterous and persistent, but don't lose sight of the fact that they have only an animal-level intelligence.
Of course this doesn't preclude multiple attacks from different creatures during your voyage, each time inflicting some level of damage to your vessel or attrition to your crew. 537
[;)
So, you're very unlikely to kill it because all you see are appendages, but if you inflict enough damage in a short enough period of time then it should give up and move on to easier prey. I have a special "morale-like" check I roll for unintelligent monster combat just to be self-consistent.
Just throwing this out there, but kraken-like creatures are what I call "edge of the map" monsters. They're generally barrier monsters, meant to be impassible or to make you pay for getting past them. They may not fight to the death, but they should be among the most difficult to kill. In this case, the reason they're difficult to kill is because you never see them.
Poisoned spears and harpoons. 164