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Research Vehicle Reaches the Bottom of the Ocean
timothy found BBC coverage of the voyage of the Nereus, which on May 31 dove to the bottom of the Challenger Deep in the Marianas Trench. Only two vehicles have accomplished this feat before, the last 11 years ago. "The unmanned vehicle is remotely operated by pilots aboard a surface ship via a lightweight tether. Its thin, fibre-optic tether to the research vessel Kilo Moana allows the submersible to make deep dives and be highly manoeuvrable. Nereus can also be switched into a free-swimming, autonomous vehicle. ... The Challenger Deep... is the deepest abyss on Earth at 11,000m-deep, more than 2km (1.2 miles) deeper than Mount Everest is high. At that depth, pressures reach 1,100 times those at the surface."
Yes salt water is very good at attenuating RF, the higher the frequency the worse it is. Have a look at http://en.wikipedia.org/wiki/Extremely_low_frequency on Wikipedia that highlights some of the difficulties, especially in relation to antenna size. Also at those frequencies you can end up with transmission rates less than one bit per second.
... the ocean might simply be good at blocking transmissions.
The ocean isn't just good at blocking transmissions. It's ridiculously good at blocking radio waves. If you work the math on this page, you can see that your basic WiFi transmission (at 2.4 GHz) will experience an attenuation of almost 1700 dB/meter! At that rate you'd get far less than a millimeter of penetration.
Even the lowest frequency short wave bands (1.8 MHz) get 46 dB/meter attenuation. It starts to get possible to receive RF when you get down in the kHz range but of course, your data rate goes to hell.
For underwater communications under a couple hundred meters or so you can use an acoustic modem. Even then, your best data rate is going to be on the order 2400 baud or less.
If you want high speed underwater communications, you gotta use a cable.