In the old days they would trail out a 5 nm cable antenna to pickup a one way EAM (emergency action message) via VLF/ELF through the seawater. The stations where at Cutler, ME, Chicago, Western Australia, and maybe more. It would be a worldwide system that deployed encrypted slow Morse code.
Today, they use SLC. The captain has the sub come to SLC depth at a predetermined lat/lon. They shine the B/G LASER straight up through seawater. The SLC satellite scans for the beam and returns a B/G LASER back.
Now they have a secure 2-way link for EAMS and routine traffic. A surface ship can also SLC the sub too, if he knows where he is. However, the captain could just use his portable FSO system (while surfaced) to talk to the surface ship secretly.
