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.

Completely impractical for SCADA systems. :)

I'm really interested in learning more about underwater ULF - can you point to any keywords, papers, or milspec prefixes to read up on?

I've explored all the ISM bands - I just want to play with something even slower/longer range whilst sailing (in international waters, of course :)).

There a little here: http://en.wikipedia.org/wiki/Communication_with_submarines

and interesting detail here: http://en.wikipedia.org/wiki/Project_Sanguine

Thanks guys.

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]).

[1] http://www.mit.edu/~millitsa/resources/pdfs/royal.pdf

[2] http://liquidr.com/prodserv/wg/gateway.html

jmah's Wikipedia links are a good start. But be warned, water penetrating ULF is outside the scope of hobbyists and even most small governments.