George said swimmers at the nearby beach made a mad dash for the shore when they spotted the behemoth.
"All the swimmers got out of the water pretty quickly," he wrote.
George said he got a closer look and determined the beast was a basking shark, the world's second-largest fish and a type of shark considered harmless to humans. He estimated the shark was about as long as his 18-foot boat.
Witness Hannah Handscombe described the shark as "an awesome sight."
She said George informing onlookers of the basking shark's identity did not comfort many of the swimmers.
"The swimmers all ran out of the sea despite knowing it was probably harmless and everyone just stood and stared," she told the Plymouth Herald.