Saltwater specifically, or just in practice?
Long AC lines in air also have a finite resistance between wires, but it is much higher and not much of a concern.
>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)
Fresh water is way worse than air, and salt water way worse than fresh.
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.
[0] https://www.homedepot.com/p/Scalewatcher-Nano-Electronic-Des...
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.