A method of measuring the effective temperature inside a sealed container having a headspace is provided. The material is comprised of a central core layer of fluoro-polymer based sheet material to each side of which a separate polymer film is adhesively attached. In some aspects of the method, a highlight color with a plurality of color components is selected, for example, white. The method can further include passing a catheter over the guidewire after the guidewire has passed through the septum. In one embodiment of the submersible watercraft of this invention, the hull comprises air bladders, thereby avoiding the engineering complexity, weight and cost of constructing a pressure hull.