I'm very comfortable with snakes, poisonous or non, but I've had two 'too close for comfort' encounters with cottonmouth's. Both had me doing the cotton-eyed Joe while screeching like a girl.
A fairly small CM was taking a nap under the bow of a jon boat and when I went to shove off the bank he gave me a 1/2 sec. warning then took a swat at my leg.
The other time I was standing in the middle of a fast riffle as a rather large one started to cross just upstream of me, I hollered at my buddy to "check it out" and when I hollered the CM stopped swimming and the fast current swept him within a foot of me.