Seems you're right, I just was searching how the military does it and...it's not great. Came across https://en.wikipedia.org/wiki/Communication_with_submarines

Seems you're right, I just was searching how the military does it and...it's not great. Came across https://en.wikipedia.org/wiki/Communication_with_submarines https://en.wikipedia.org/wiki/Communication_with_submarines . So perhaps the best you could do here are something like emergency buoys that either one can record data on or perhaps automated recording some stats/location/whatever that can be released in an emergency and send a broadcast once topside?