1. An anti-collision warning system for marine vehicle comprising: at least one optical sensor covering at least partially the horizon of the marine vehicle, and supplying images; an image processor that searches in real time in each image supplied by the optical sensor for groups of adjacent pixels according to the light intensity of each pixel of the image, that assesses the afterglow of each group of pixels in successive images, that determines groups constituting objects visible on the surface of the sea, according to their afterglow, and that determines position information of at least one of the visible objects; and anti-collision analysis control that periodically calculates the evolution of the position information of at least one visible object, and that assesses a risk of the marine vehicle colliding with the visible object according to the evolution of the position information of the visible object.