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
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.
