Abstract: A pressure vessel capable of withstanding elevated external hydrostatic pressures is provided, comprising a tubular casing capable of withstanding hydrostatic pressures having an internal cavity and at least one end opening permitting entry into the cavity. An end closure seals the end opening and defines an external joint between the closure and the pressure vessel casing upon sealing of the opening. The end closure has an outer circumferential surface which is coextensive, that is flush, with the adjacent circumferential surface of the pressure vessel casing. At least a portion of the pressure vessel casing and closure are encapsulated by a seamless tubular sleeve of heat or air-shrinkable material which covers the entire extent of the external joint and is heat or air-shrunk about the vessel to seal the external joint. There is a layer of sealant in the form of an adhesive or a mastic, between the shrunk sleeve and the vessel.