I've held a full-size python, and on another occasion a different type of snake. The latter was way smaller and it was coiled up on my hands so it was adorable in a way. I've also held a baby gator twice. And yes, I was slightly terrified during each of those experiences.
I'll probably never swim with sharks though. I go to the beach regularly and I don't need that stress in my life. Last time I was at the beach I thought the shadow casted on the water from a skyscraper was a lovely little creature out to get me. I was fooled for only a split second, but I sure as hell swam back to more shallow waters where I could plant my feet on the sand.
The joys of living in America's Australia.