Which American actor worked as a lifeguard, but on one occasion, was unsuccessful at rescuing a man from drowning; this would haunt him for the remainder of his life?

Ronald Reagan made a similar statement. Unfortunately, newspaper accounts in the area where he was a life guard never mentioned a drowning when he was working there. Nor is there any real evidence (other than Reagan himself) that Reagan saved 70 some-odd lives as he had claimed. But it made for a good story.

DG,I have a different answer than R. Reagan.

Hi Mozart,
It's Telly Savalas, better known to millions worldwide, as Kojak.
Savales was a world-class poker player, held a degree in psychology and was a motorcycle racer.
He released two chart topping singles, as well as being an accomplished actor and writer.
However, long before this success, and on graduating from High School, Savalas was a lifeguard in the early 1940's. He unsuccessfully tried to rescue someone from drowning, and this failure would haunt him for the rest of his life.

Wikipedia

"Who loves you baby". That's the guy! Thanx dance girl.