Yeah, DC is pretty much a need for under(salt)water, maybe except for very short distances.

For offshore wind farms, typically only very long export cables are HVDC, or long interconnector cables. Inter-array and export cables up to around 100km or so are AC.

Traditionally, or at current times?

https://en.wikipedia.org/wiki/HVDC_Troll and

https://en.wikipedia.org/wiki/HVDC_Visby%E2%80%93N%C3%A4s

come to mind.

Those two examples are rather small. Not even at the level of supergrid interconnectors proposed, or built elsewhere.

Technology improves and its availability and costs change. In this case https://en.wikipedia.org/wiki/Insulated-gate_bipolar_transis... which appear everywhere from induction cooking fields/plates in household appliances to power electronics in electric cars and locomotives, upwards to so called smart- or super-grids(in their endpoints/substations). I'm sure there are even more applications of the enabling IGBTs which I'm unaware of.

Saltwater specifically, or just in practice?

Saltwater is a very poor dielectric between wires (it is a high resistance conductor actually). An AC line would have a low efficiency because the dissolved ions transport a leak current between the wires. Around a DC line, electric field just generates an ion gradient once when powered on (and assuming the insulation doesn't experience electrolysis).

Long AC lines in air also have a finite resistance between wires, but it is much higher and not much of a concern.

Does a long distance DC line even need a negative wire, or can you just connect it to ground at the destination?

They can work without a return wire. It sounds like this system is designed to be able to work that way in case of faults:

>The use of two HVDC cables will provide the link between all four sites. The use of two cables constitutes the bipolar operation of the project. A return path through earth, however, is provided in order to allow for the reinstatement of rated or half-rated power transfer through converter switching, in case of faults and disconnections of one of the two 500MW elements of the converter, or one of the two HVDC cable links.

(https://www.euroasia-interconnector.com/wp-content/uploads/2... p17)

Upvoted, and let me add: what a fantastic comment - deep and clear and easy to grasp.

The more conductive the surroundings of the cables, the more losses you'll get by induction.

Fresh water is way worse than air, and salt water way worse than fresh.

worse as in more conductive

Worse as in larger losses, that is the consequence of more conductivity, so yeah, you can read it that way too.

I believe AC current creates magnetic interference that is greatly magnified in water - causing "drag" on the transmission if you will.

What it creates is induction currents in the surrounding water, with dissolved ions being pulled this way and that, against drag of the water, as the fields vary.

On DC lines, fields don't vary (much) so induce negligibly little currents.

In fresh water, there are fewer dissolved ions, but not none. Losses are less.

In perfectly pure water, small losses would come from swinging water molecules to point this way and that. Raindrops hanging on AC transmission line wires consume negligible power.

Pure water has self-dissociation, so it always contains small amounts of ions (that's why pure water has a pH and pOH of 7). Butthe dipole moments of H2O molecules dominate theinteractqiqo

I suppose that's what things like this [0] do?

[0] https://www.homedepot.com/p/Scalewatcher-Nano-Electronic-Des...

Those purport to operate on the ions themselves, supposedly to favor one kind of crystallization pattern over others. In lab conditions, a permanent magnet is said to work equally well, presumably interacting with the ions as they flow past.

From what I have been able to determine, nobody has shown that such a gadget works with any reliability. I.e., it might work under certain circumstances, but there is no way to know if your water and pipes match such a circumstance without buying. And, the prices quoted seem badly excessive. There is probably not more than $5 worth of parts in there, if in fact there are any.

Personally, I would not buy one. You could experiment with a permanent magnet, but it would be hard to know if it was helping or making it worse.

A somewhat similar sort of gadget is supposed to actually work, on diesel immediately before injection into truck engine combustion chambers, to produce more complete combustion.