Boarding systems for retractable ladder boats comprising a stern platform and a retractable ladder; a water-tight casing-sheath for said ladder, equipped, inside, with sliding guides and incorporated in the stern platform, comprises a plurality of sliding slides, housed and limited inside the casing-sheath. The stern platform favors a receiving space, provided inside or outside, for receiving the casing-sheath and its optimal fixing. The sliding slides have a free stroke on the entire length of the sliding guides since such free stroke is limited at the ends of the sliding guides opening out on the opening port, through locking elements that allow the sliding slides to partially slip out from the sliding guides. The casing-sheath is retractably housed inside the housing space, formed in the stern platform.