The boater, Iouri Lizourn, was last spotted about 100 yards off shore and about 100 yards north of the Ben Franklin Bridge on Wednesday night.
It was around 10:30 p.m. when Lizourn's son called 911 to say his father fell in the water.
He was wearing dark clothing, making it difficult to spot him on the nighttime water.
A Coast Guard rescue boat, a helicopter and state police troopers are participating in the search.
