Forget about the method I just mentioned, this appears to be better (although I haven't played it that much yet)
Open the program files\silenthunteriii\data\menu\cfg directory and edit the maps.cfg file. Change the settings on the navigation map to:
[Map0]
MapID=Navigation_map
Position=0,0
Resolution=1024,656
MajorSqrs=4,2.4
MinorSqrs=5,5
PixPerUnit=50
ZoomLevels=1,2,5,10,20,50,100,200,500,1000,2000,40 00,7000,10000,20000,40000; meters per pixel
InitialZoom=20000;originalmente 500
SymbolZoom=20000;units are drawn as symbols from this zoom up originalmente 10
GroupsZoom=40000;units are drawn in groups from this zoom up originalmente 100
CityZoom=10000;city names are displayed from this zoom down
KriegZoom=2000;krieg labels are separated displayed from this zoom up
HarborZoom=50;harbors will be displayed from this zoom up
Having done so, open the program files\ubisoft\silenthunteriii\data\submarine folder, then choose the folder of your favorite sub, and edit the NSS_UboatXX_shp.TGA file, making it into something invisible (that's to say, just make the whole picture a transparent blank).
There you go, the only way to see your sub is to zoom way way out, wich won't help you much for the finer details of navigation, forcing you to rely on alternate methods.
Now, about faking the sextant, any time you deem that a decent navigator could calculate your exact position, press and hold the ctrl button while clicking with the mouse on a certain point on the nav-map, you'll notice that the nav-map will shift... your boat (even though you can't see it) is right there, under your mouse cursor, so mark that spot for later reference!
As I said before, I need to play with this for a while to see what type of unfortunate side effects this has... I'll report any findings later.