Acoustic Transmission
Sound travels far in water, and underwater loudspeakers and hydrophones can cover quite a gap. Apparently, both the American (SOSUS) and the Russian Navy have placed sonic communication equipment in the seabed of areas frequently traveled by their submarines and connected it by underwater communications cables to their land stations. If a submarine hides near such a device, it can stay in contact with its headquarters. An underwater telephone sometimes called Gertrude is also used to communicate with submersibles.
Read more about this topic: Communication With Submarines