A friend of mine, let's call him Chuck, who is somewhat crazy, tells this story. Knowing him I believe it!
One day, Chuck was out diving with friends off Davenport, about half way between Half Moon Bay, and Santa Cruz CA.
He was swimming along, minding his own business, trying out his new $80 flippers. A seal grabbed one of his flippers. Annoyed, he kicked his foot up to shake the seal loose. This flipped HIM up under what he describes as a boxcar with fins. His "seal" was a Great White Shark!
The shark didn't like the taste of rubber and spit out the fin. He boosted for the surface stuck his head up and said "HELP!" Head back down, look for shark. Head up: "SHARK!" At this point he sees everybody on the boat franticly running to and fro. He and the boat manage to get together.
Later he found out that some local fishermen were in the habit of cleaning their catch in that area.
Soooo, the shark came up beside the boat looking for a handout. Finding none it then swam off in his direction. His friends said it was as long as the boat, 12 feet.
OK, now fast forward a month and a half. Chuck is visiting friends in San Diego, They go out for a dive. There are Basking Sharks, a harmless filter feeder, in the area. So his friend who knows about the earlier dive says, "Hey, Chuck! Wanna swim with the sharks?"
Slow Learner Chuck says "Sure!"
Soon, he's in the water. He sticks his head up and can see the dorsal fin of a shark approaching him. Oh goody! Dunks his head and looks. The water is too murky. Head up. Fin still coming. Head down, too murky. Then...
Out of the gloom a mouth "big enough for me to stand up in in any direction" suddenly appears.
His friend says it was the funniest thing he ever saw. Chuck came straight up out of the water, walked 10 feet across the surface and sank back down...
