Floating containment booms for containing petroleum and similar product spills on a water surface such as at sea, in harbors, rivers and the like, and methods for forming the same. In accordance with the method, an elongate sheet of flexible material and components for mixing to form a plastic foam or a foam forming or expanding material together with apparatus for forming the containment boom therefrom are stored for use when required. In operation on a boat, the apparatus forms a loop or opens a preformed loop or loops in the material, at the same time injecting the foam forming material in the loop so formed to expand and fill the same with foam at the same is played out from the boat while traversing the region to be surrounded by the oil boom. The portion of the elongate sheet opposite the loop is preferably weighted, either prior to use or as the oil boom is formed and played out from the boat. Various embodiments are disclosed, including embodiments having a sea anchor-like provision on the lower part thereof so that the portion of the elongate sheet which hangs down under water under the foam filled loop is restricted from rising and given a large apparent mass in agitated water by the sea anchor-like action thereof.