One of the reasons to use quantum gyroscopes is that will be accurate to within 1m or so after a day long underwater traverse of 1000 km or more. Communication while fully under still sucks though.
Powerful lasers, as used by laser depth sounders, will penetrate water to an 80m depth [1]. Might be possible to do a laser link directly to an overhead UAV or satellite? If they are following regular shipping lanes, it might be possible to have communication buoys (or nodes on the seafloor connected by fibre/cable) at regular intervals along the route, with laser or sonar communications to passing submarines. It would be a bit like "sections" of track in railways, with vehicles checking in as they enter/leave each section.
femto|4 years ago
[1] https://www.navy.gov.au/ran-aviation-history/laser-airborne-...