An obstruction positioning in a recess between an adjacent pair of security bulwarks mounted outside a perimeter railing of a vessel or marine installation is provided. The obstruction comprises a rear end for positioning towards the perimeter railing, a front end for positioning away from the perimeter railing, and a top side, an underside and a pair of opposing lateral cheeks separating the front end from the rear end. The obstruction comprises a connection facility for connecting the obstruction to at least one of the perimeter railing and the adjacent pair of security bulwarks. The top side slopes downwardly from the rear end towards the front end during use of the obstruction between an adjacent pair of security bulwarks, so that a hand of an unauthorized potential boarder placed on the top side of the obstruction will tend to slide off it in a seaward direction.