Believe me, that's a positive part of the whole noodling experience. From Wiki:
To begin, a noodler goes underwater to depths ranging from only a few feet to up to twenty feet, placing his hand inside a discovered catfish hole. If all goes as planned, the catfish will swim forward and latch onto the fisherman's hand, usually as a defensive maneuver in order to try to escape the hole. If the fish is particularly large, the noodler can hook the head around its gills.
Although few confirmed deaths have been recorded in the recent history of noodling due to its obscurity, there have recently been unconfirmed reports of a noodler from the mid-south reaching into a catfish hole, only to be greeted by a group of water moccasins which fatally bit him.
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A guy I work with said he used to do it all the time as a kid. Personally I ain't sticking my hand in a hole in the water.