Don’t think for a second that this couldn’t happen, wouldn’t happen, or hasn’t happened, in marathon swimming:
“Marathon Man” (by Mark Singer). In The New Yorker, August 6, 2012.
Related question: If Diana Nyad touching the boat during her feeds hadn’t been captured on video, would we have ever known she was doing this?
Also See: Two Golden Rules of Open Water and/or Marathon Swims (LoneSwimmer)