Here's an interesting story asking the question: Are squid vicious? (Site may require registration.) Specifically, he's talking about the Humboldt squid, the "Arnold Schwarzenegger of the squids." Large numbers of these squid have taken over in the Sea of Cortes. Here's part of the story:

"At one point, as Kerstitch was underwater, a passenger stood on the deck fighting a 12-foot thresher shark with rod and reel. At about 30 feet, Kerstitch caught a glimpse of the shark -- and of a large squid flinging itself at the struggling predator.
"The squid quickly dashed away but not before removing 'an orange-sized chunk of flesh from the side of the shark's head with its powerful beak,' Kerstitch told me not long after the encounter. Other squid then turned on the diver. One latched onto his fin and pulled him down. Curious, Kerstitch let it pull, which may have provided this sense of opportunity.
"Kerstitch kicked and the first squid let go. But another attached itself just above his shoulders. 'It was like somebody was throwing a cactus on my neck,' he recalled.
"He struck the animal with his dive light and it let go, taking a gold chain he had been wearing. Another squid then wrapped its arms and tentacles around his face and chest. He buried his fingers into the body of the squid and began to pull. It slid to his waist and let go, taking his decompression meter.
"Kerstitch escaped and flopped back onto the boat, glad to be alive but burning with nasty lesions. 'These could eat one of us in a New York second, if that's what they wanted,' says Roger Hanlon, senior scientist at Marine Biological Laboratory in Woods Hole, Mass. 'Two gnashes of that beak and your wrist could be gone.'"