Amir K. just wants to be prepared:
Assuming this is a 6 foot 100 lbs humboldt squid, it goes red, all of its arms point together and it's about to shoot straight at you with it's 2 long feeding arms. What do you do if you are in the water? What is the best way of repelling an attack or destroying it? What are it's weaknesses?
Oh Amir! The first two "its" were perfect. Why'd you have to go and ruin my good opinion of your grammar by adding apostrophes to your second two "its"? But you know what--I'll answer the question anyway.

First of all, get out of the water. There is never a good reason to stay in the water when being attacked by an aquatic animal--unless the animal in question can also fly. Well . . . on second thought, you'd better not get out of the water. 

Next, remember that squid are visual predators, but rather hard of hearing, so feel free to shout for help as loudly as you like. The squid won't know what you're doing, and your backup will have the element of surprise.

Now, about that backup. Dolphins, whales, tuna and sharks all like to eat Humboldt squid, so they're on your side. If you squeak Dolphin, feel free to bust out a high-pitched S.O.S. But if it's been a few years since you practiced and you're nervous about your contractions, don't try it--dolphins are notoriously picky about grammar. You'll be better off summoning a shark. They don't care how you decline your nouns as long as you leave a trail of blood in the water.

That's right, you'll have to let the squid get a hit in, and you'll have to bleed more than just a drop. It may hurt, but the saltwater will soothe the bite. Meanwhile, the squid thinks he's winning, while you know your savior the shark has smelled your plea for help and is on the way.

But if nobody comes to save you and you get tired of being chewed on, remember the squid's greatest weakness is its skin. One little tear in that tender epidermis can lead to infection and death in just a few days. That's also about how long it will take a Humboldt squid to consume a fully grown human, so you can pass the time by betting with your buddies on the boat on whether you or the squid will die first.

Hat tip to Julie S. for pointing out this gem on Yahoo! Answers. If you want a more serious assessment of the likelihood of being eaten by a Humboldt squid, go here. Otherwise, happy swimming!