In loving, living memory, John Melançon 1928 – 2007
Helping clean Grandpa's boat, I was dumping some relatively benign cleaning water out of site of the harbormaster. This involved (not exactly out of necessity) climbing down the side of the boat with one hand holding the bucket. With hand on the top railing and toes on the first ridge, I lowered myself so that the toes were on the lower ridge. I estimated that I could let go of the top railing and grab hold of the cleat quickly enough to not fall backward into the water... and I was right.
However, in keeping the eye on the cleat, my toes left the ridge, and I fell, but hung on easily with the one hand, with my bare feet and the bottom of my panted legs in the water.
I got my feet back on the ridge, dumped the bucket, and climbed up (easier with the bucket empty).
So it *is* possible to catch oneself from falling with one hand. Probably not after any speed is built up though. I'll try that out some other time.