Still looking into ways which the real submarine might be made (with a touch of salt due to its outlandish design) as this helps to make the model look more realistic. Or as realistic as a fish style submarine complete with fins can look! Metal spinning might be used on the dome of the head. Wikipedia explains it pretty well:
http://en.wikipedia.org/wiki/Metal_spinning
This is how the tops of rockets are made! Also bells and cookware can be manufactured in this way. You could use a hydraulic spinning lathe for the job. These are the professionals at work at Winward engineering ltd: http://www.winward.co.uk/about%20us%20new.htm
This would create a completely smooth finish, or one with slight rings left by the lathering tool. Looking at the shape of the head with the current design I think that it is more likely that one would used a beating metal technique all over as its not a complete dome.