Re: For those who wonder...
"AC was just more efficient with less line loss."
this mostly has to do with distribution locally, not over distance. the problem with DC is that you can't EASILY convert a high voltage (like 100kv) into a line voltage for home use (let's say 220 or 110). So the generators had to make DC at 'what you get at the wall socket', meaning very high current and lots of loss.
However, if you send DC over distance at 100kv (let's say), then all you need is a motor/generator setup on each end. In fact, an MG can go BOTH directions. A DC line under the channel could then be a bi-directional power connection. It's probably why they do it "that way". Also a DC setup could have surge capacitors and batteries on it to help absorb transients etc. etc.. Sort of like submarine power, which still has backup generators and batteries very similar to old WW2 diesel subs. Hey, it works!
The motor/generator [for those who do not know] is what it says on the tin. It's a motor on one side, and a generator on the other side. The mode of operation is essentially based on how the regulators are set up to work. A generator on the DC side will regulate voltage. A motor on the DC side will regulate speed. Similarly, if the AC side is operating as a generator, its frequency regulates the DC side. If it's operating as a motor, the voltage and current on the DC side determines the loading. Both the voltage and speed regulation modes would have appropriate settings curves, so that load is shared and regulated properly, and you adjust the regulator's curve point to maintain 50Hz (or whatever), or 'n' volts on the DC bus.
Also in this way you can parallel the MG sets, so that you can take one or more off line for maintenance [yeah those DC brushes will need to be changed out periodically, and the contacts cleaned, etc.] but with proper care and maintenance, an MG set will spin for DECADES without major problems, and operate very efficiently. They really do work very very well, and are probably the most reliable way to transfer very large power levels between AC and DC, while simultaneously allowing bi-directional flow.
Additionally, you could use a motor/generator for standard conversion - a 60Hz unit on one side, a 50Hz unit on the other, with an appropriate number of poles on each so that they spin the same speed. Often they're used for 400Hz systems in that way.
Another advantage of using DC to transfer power is that you separate the line frequency controls on either end of the distribution. I assume this is what they wanted to do here. And when you're putting power lines under water, maybe the inductive losses are much higher with AC than with DC. I'd guess that's it at face value, without researching even.