Submarines communicate using extremely low frequency radio transmissions to penetrate water. They have a bandwidth of 2-3 characters per minute, so the messages are almost exclusively orders to surface and switch to standard satellite based communications. Oh, and the transmitter requires a very unique piece of land with low ground potential that only exists in a dozen or so places worldwide.
I was familiar with Project Sanguine, but had hoped there were less ambitious public projects I had overlooked (perhaps closer to VLF which operates <30m seawater depths).
After reading about the Navy E-6B aircraft, which trails a 5km antenna behind it to communicate with subs, I had presumed modern 'submarine -> other underwater radios' were akin to large commercial fishing trawling nets - or the really long antennas were packed into hilbert curves and epoxied to the hulls or something.
Anyhow, it seems 'acoustic modems' using 'CSMA' [1] are the norm for commercial underwater ROVs (such as James Gosling's 'wavegliders' [2]).
Completely impractical for SCADA systems. :)