It happened on North Delaware and Linden avenues in Philadelphia's Torresdale neighborhood at approximately 5:30 p.m.
The fire department was called to the scene at the Linden Avenue Boat Ramp for reports a woman fell off a Jet Ski and swam to a "Jersey island."
She was rescued from the water just before 6:30 p.m., according to police.
The 51-year-old was transported to Jefferson Torresdale Hospital after being rescued. Officials say she is in critical condition.
There is no word yet on what caused the woman to fall off or lose control of the Jet Ski.