Yfke Sturm suffered a fractured skull, severe spinal injuries and remains in a coma following the accident near the island of Ischia.
The London-based 33-year-old is being treated by specialist doctors at a hospital in Naples.
She is understood to have fallen off a jet surf, which is a kind of motorised surfboard, and was attempting to swim to safety when she was struck by another jet surf rider.
Emergency services were called and Sturm was rushed to hospital with serious injuries. Doctors at the hospital are concerned she will be left with permanent paralysis due to the spinal injuries she suffered.
The 33-year-old has walked the catwalk for popular international brands such as Ralph Lauren, Calvin Klein, Victoria's Secret, Marc Jacobs and Dior. She has also appeared on a number of Dutch television shows, including hosting Holland's Next Top Model.
Sturm recently gave birth to a boy whom she named Alexander.
Her representatives have declined to comment on her condition.