You don’t need a submarine to cut a cable. Cables are sometimes damaged by ordinary ships anchors. It’s not difficult to imagine a device, designed to cut cables, being dragged by an ordinary ship doing this damage.
But then again.....
Egypt’s Transport Ministry, in a statement, said that footage recorded by onshore video cameras of the cable locations showed no maritime traffic in the area when the cables were damaged. “The area is also marked on maps as a no-go zone and it is therefore ruled out that the damage to the cables was caused by ships,” the statement said. - http://www.thehindubusinessline.com/2008/02/06/stories/2008020651700400.htm