Again, this is what we know about the state of the art 20 years ago:

For receiving (very very slowly) there is VLF, as the other comments have said. Can be done underwater. Think 160 character tweets that take minutes to arrive.

One option is to release a buoy that floats to the surface, waits a while and then transmits the payload. This allows the submarine to be out of the area when the transmission happens. This gets you "don't know where the sub is" but doesn't necessarily get you "didn't know there was even a sub there".

Alternatively, the submarine can come to periscope depth - so underwater but able to raise an antenna above the surface (or maybe close to it). Then something sophisticated like direct laser communication to a satellite can happen, allowing for high data rate bidirectional comms. Assuming you aren't detected by coming up, then this SHOULD allow for completely undetectable comms.

Very low frequency radio: https://en.wikipedia.org/wiki/Communication_with_submarines

They can trail an antenna behind the boat underwater that receives low frequency radio waves carrying short signals that basically tell them to come up to a higher depth to receive a longer message.