I have not seen one and do not ever care to see one. Yet one of my friends was diving off the California Coast, North of San Francisco Bay, which is well known for them, and had a terrifying experience. So much so that he quite diving in the waters here. The story goes something like this. They jumped out of the boat in about 50 feet of water, with approximately 15 feet of clear visibility. Started their decsent, and when they reached around 30-35 feet a huge shadow passed over them. When they looked up, they saw a giant 18 foot Great White cruising over them. They decsended to the bottom (Too fast as he put it) and held position. Every couple of minutes the shark would pass overhead, but would just come out of no where due to the visibility. The shark was very consistent and after being on the bottom for over 40 minutes they decided to make a beeline for the boat before they ran out of air. Fortunately they made it safely to the boat and home to his wife and kids. My friend does not seem to embelish things and it came up in passing conversation, so I tend to believe him. Especially since it took an adrenaline junky fireman and scared the poop out of him.