The original report came from the captain of River Princess Cruises, the Nottingham Post reported Friday. He spotted what looked like a body early Tuesday afternoon in the Trent in the suburb of Colwick.
Police officers who responded drafted Alex Bell, who lives on a canal boat in the area and was out in his motorboat, to help.
"She was so motionless and pale," Bell said. "She was just floating in the water and the policeman thought she was dead.
"I didn't really want to look but then I saw her flicker her eyes and said 'she's alive' -- and then she opened them."
Mat Mitchell, a police constable, said he also believed the woman was dead. But he called to her to make certain.
"In one of the most bizarre moments of my career she just opened her eyes," he said.
Mitchell shed his clothes and jumped in the river wearing only his boxers. He pulled the woman to shore.
She is reported to be doing well at the Queen's Medical Center in Nottingham.
There's no word on how the woman got into the river.