I've never heard of a wild orca killing anybody, but I have heard of them tossing sea lions larger than most of us high into the air, and in a wetsuit I look a little like a big, fat sea lion. Plus it's a wild animal, doesn't have 'morals' as we understand them, and there's no particular reason to think it's got an abstract appreciation for 'sentient rights' or of us as sentient beings. In other words, while it may not be in an orca's nature to harm a human, they are intelligent, apparently can be playful, and if one's in a mischievous mood to 'just screw around' and play with a human, I don't know any compelling reason why it couldn't.
Oh, long as we're considering long shots like giant squid & orcas, I now recall a DAN Alert Diver magazine article on someone diving with a polar bear, and it came after him! It was race for the bottom for awhile! I believe that was a researcher, not a recreational diver, but still... Article -
In the Company of Polar Bears - Amos Nachoum. And
here's a link to a YouTube video of a giant squid at Toyama Bay with a guy in the water with it.
On the alligator issue; I wouldn't want to be in the immediate vicinity of a large alligator, but if the water were clear, it weren't aggressive, viz. was good and it were stationary, I might go in the water. People dive around them without knowing it; I've dove Alexander Spring in Florida and only saw a distant 3 or 4 footer at the surface, but an old friend of mine who lives in Florida and got there some mornings before the crowd told me he'd seen a 10 footer. With all that vegetation, who knows what's around?
Big crocodiles are another story. I'm not sure what the 'cut off' size is. There's snorkeling with American crocodiles (XTC Dive Center in Xcalak, Mexico offers this -
free access Undercurrent article), and I think even diving (
Jardines de la Reina article in DAN's Alert Diver - by Stephen Frink).
Richard.