A boom (1A) is described, especially for containing oil spills and/or other similar pollution at sea, in rivers or on water surfaces, comprising an elongate body consisting of an element having positive buoyancy relative to the surrounding medium, so that a freeboard (5) is provided, and a hanging skirt (6), which boom (1A) is of the type that is usually lowed behind towing vessels or is moored (2) so that the boom (1A) basically forms a U-shape having side arms (4) that define a front opening (10) between them, which arms extend from a rear area or apex (7) when the towing vessels or mooring (2) pulls at the free ends of the arms of the boom (1A) in order to tow the boom (1A), and wherein the skirt (6) is provided with a draught that varies from a minimum size or depth in the area at the boom apex (7) to a maximum size or depth at the free towing ends (15) of the side arms (4) of the boom (1A).