Simple Sally's shark post reminded of another deadly creature in the ocean, the big and ferocious Humboldt squid. These creatures are intelligent, strong, aggressive and can easily overwhelm a diver who is unprepared. They will attack sharks and sea creatures much larger than themselves. They are known to have killed Mexican fishermen.
interesting article here with pics:
http://diver.net/seahunt/fend/f_scottc.htm
video of divers with the squid.
http://209.242.151.5/seawolves/DancingWithDemons.wmv
Article about French sailers whose boat was attacked by a giant squid