Very similar to an accident that happened in our area about 20 years ago. A biker waded out into a pond at a campground in Andulusia AL to cool off when an 8 ft gator grabbed him and took him under. The gator was twisting his arm as he held the man under. The husband of a woman that worked with my wife was a friend of his and visited him in the hospital-- he said he was praying that his arm would come off, because he knew if it didn't he would drown. They were able to recover his arm and re-attach it.
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Skeet