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Slow comms via sound and ultra-low-frequency rf.
When faster data is needed pop loose a tiny transponder to float to the surface with a thin tether of optical fiber back to the boat. The rf device would only need to be the size of a marble... data transfer via the large NRO satellites in geo orbit 23,000 miles out. Power output from the device would be far less than a milliwatt since the big NRO birds can hear anything and so nearby ships would be less likely to notice the signals.
Anyhoo, that’s my guess as a computer and radio geek :-)
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