The "advert" circulating on social media sites claims that updating devices with the operating system installs a "smart switch" that cuts off the phone's power supply when water is detected.
This, it claims, "prevents any damage to your iPhone's delicate circuitry".
The advert looks remarkably similar to an authentic Apple advertisement, with the same plain white background and minimalist font and style.
It seems some users have been fooled into dunking their expensive gadgets into water to test out the promised feature, only to render their devices useless.
One user wrote on Twitter: "Whoever said iOS 7 was waterproof **** you."
Another wrote: "OK whoever said iOS 7 is waterproof GO **** YOURSELF."
The prank idea is believed to have originally started on the controversial forum 4Chan.
The iOS 7 software was released last week, and is available as a free upgrade for newer models of the iPhone and iPad.
It provides a series of new features including an overhaul of the interface.
It was released shortly before the new iPhone 5S and 5C models went on sale.
The 5S features a fingerprint unlocking device for extra security. However, a group of German hackers has found a way to bypass the measure.
The group, known as the Chaos Computer Club, says a fingerprint of the phone user, photographed from a glass surface, is enough to create a fake latex finger that could unlock the phone.
Apple has not yet commented.