Can’t tell if trolling or not.

The cables have water barriers so the conductors + insulation remain quite dry.

I believe the parent meant that the alternating current creates an electric field which is then absorbed by the water, leading to energy loss. This happens to some extent to all AC transmission, but I don't know enough to say if seawater+AC is a particularly bad combination. It would make sense though, because it's more conductive.

EDIT: brandmeyer's comment explains it well: https://news.ycombinator.com/item?id=16903431

Yes, that's precisely the thing I was referring to. Wasn't sure about the correct terminology (not an EE) so I didn't try to make the comment sciency-souding :).

I work installing these cables and never heard this discussed, maybe at the voltages / transmission subsea cables work at (220kV, 1000s of amps) makes this effect negligible?

I honestly don't know. I'm not an EE, I only reported on what I've heard reported on the topic (and that passed my "basic physics sniff test").

Are they AC? My impression was that practically all vaguely modern undersea cables are high voltage DC.

It depends, when talking about interconnecting cables between countries, yes the majority are DC, both to minimise transmission losses and also to tie together grids working at different frequencies....

In terms of km installed I think it will be HVAC just from the sheer number of offshore windfarms built over the last 10yrs in Europe. These things can be huge - 60km in length, 220kV ~1500A....

Salt water is a conductor. Two conductors separated by insulation make a capacitor. This extra capacitance makes AC less efficient.