I have now found the location of all GXW u-boats' skins (they are superb! The boats look well maintained and do not have weird paint schemes- very classy!). If one reads this and would also want to do the same- copy folders "Objects" and "Submarine" and paste'n'replace these folders into your original SH3 directory.
Also I was not able to find these particular skins in the Downloads section, but I prefer them to everything that i saw there.