The national competition authority, AGCM, found Apple's claims did not hold up under real-world conditions.
Instead, the water resistance claims were valid only with pure water in laboratory conditions.
AGCM also said Apple not covering water damage under warranty, despite claims of water resistance, was an "aggressive" practice.
The advertising at issue promised iPhones were water resistant at a depth of up to 4m (13ft) for 30 minutes.
The claim was made for a range of phones across several years, from the iPhone 8 to the iPhone 11 and their variations.