Under water submarine communication

There are a couple of methods for UW coms -

1. Underwater Telephone - essentially a telephone set that uses hi-frequency sonar to communicate with another vessel that has the same equipment. Modern Scuba divers commonly use equipment based on this decades-old Navy technology. The UWT is the primary method for short-range coms between vessels.

2. ELF Radio - While the process is classified, boats can receive (if conditions are right) ELF coms. There has been a lot of BS "concern" voiced by animal rights activists over such communications, but that's exactly what it is - BS. ELF is rarely used, and the amount of low-frequency sound generated by normal commercial shipping, underwater geologic movement, wave action, etc., is enough to deafen any whale or similar sea critter.