Very poorly. The 27MHz radio used here won't penetrate more than a meter or so. GPS doesn't work through more than some inches. But like most things, the lower the frequency and higher the amplitude the farther the penetration. Acoustic modems work better under realistic constraints, though with a few exceptions you need the line of communication to be rather vertical if you're talking very long distances (hundreds or thousands of meters) because the speed of sound in water varies by pressure/temperature/salinity so the signal path curves up back toward the surface (from Snell's law).
Sonar, which of course works by sound, has issues because of the speed variability you mention. However, IIRC (and in salt water) the range of an active sonar at the right frequency (lower is better for range) is on the order of 20 thousand meters. That said, a submarine can hide from sonar under a thermocline, assuming the sonar is above the thermocline.
Absolutely could! Some do this. It depends on your chosen operational constraints. Keep in mind that neutrally buoyant cabling isn't particularly compact, so it creates its own set of problems.
> You can't go infinitely deep anyway, because eventually the pressure will penetrate the seals.
This is actually the least useful reason why you can't go infinitely deep. For exploration you will basically always want to be near the bottom of the body of water because mid column is just extremely boring, so you will want to make sure that your pressure tolerance allows for that. We just haven't found any infinitely deep bodies of water yet.
Terrible (for 2.4/5GHz). They seemed to have worked around there not being a lot of hobbyist receivers by buying toy submarines that use lower frequencies like 40MHz and used the boards from there, nifty but seems very limited.
Doesn’t work on any unlicensed frequencies. 2.4Ghz and 5Ghz are absorbed by water. Militeary submarines use VLF frequencies. With a mile long antenna. So tether is the only real practical method.
Please point them out. All the UUVs/AUVs I've found are either tethered to a buoy or completely autonomous and only transmit when the surface. In all practicality there is no completely untethered ROV on the market. Acoustic controlled UUVs are still line of sight.
Radio communication under the sea is not an attractive option for experiment by the radio amateur as it requires the use of very low frequencies, large antenna systems and very high powers.
Fresh water lakes and rivers have much lower electrical conductivity than the sea and underwater transmission distances (or depths) up to 30 metres appear feasible using the [then-not-now] lowest frequency amateur band of 1.8 MHz. Even larger distances (or depths) could be achieved if a lower frequency band allocation were made available.
However, that was written in 1987 and from a cursory historical review of https://en.wikipedia.org/wiki/ISM_radio_band it seems that exactly around the late 1980s time that article was written, the still earlier-suggested lower frequencies within the ISM bands were popularized and confirmed by various national regulators and there are now commercially available ISM-band transceivers available, eg. https://hoperf.com/modules/rf_transceiver/index.html
You can use directional (high dBi / high gain) antennae at one or both ends of the link, however this adds substantial complexity and generally negatively affects the vessel hydrodynamics.
Let's say a sub delivering critical medicines to peoples in need is supposed to cross an ocean, it probably wants GPS at least sporadically (for inertial navigation error correction) and AIS (to avoid collisions with cargo ships in busy shipping channels) which means it's operating at least near-surface most of the time anyway which means a radio comms tower is likely the best option, and the question is moot.
OTOH, if a sub is launched from a research vessel, it's cheaper and easier to use a tether. If it's just for a hobbyist, then the commercially available ISM transponders are likely adequate. So as per the 1987 comment: it seems in-water long distance comm's is really the domain of the military due to their unique requirement for deep-water stealth and the corresponding need for unlicensed low radio frequencies. For the rest of us, use ISM-band, a towed surface relay (also stealthy anyway), a tether, or just surface for comm's.
