Opening the cable to sea water would be a quick signal to the Sovs that something was wrong.
Of course, sub could have brought along a small chamber in which to work along with a man or two, brought cable in, pumped it dry, spliced in, resealed it, etc....
Ya...gud!
No, it certainly could not have been a 'cut / solder connections' type ops.
Just really pisses me off that they knew nothing about it - did not have, nor probably still do not, have the know how to detect something like that.
Until our traitor told them about it.....