Re: Processing seawater
Argh. More fundamental misunderstanding of capitalism.
Capitalism is descriptive. "I value X more than Y, so I am willing to pay more for X." Capitalism exists even in the deepest darkest socialist economies. (AKA Black Market).
If we as a society value keeping lights on more than having them off, we will find a way. We may not anticipate the exact way, but eventually someone will find a way, and allowing them to get filthy rich is actually a small cost in comparison to the benefit. Punishing people for coming up with clever solutions is not effective in the long run, which is why socialism tends to be less successful than a society that encourages capitalism.
Good example IRL. I know somebody that knows where a mineral is. It is in the control of a socialist economy. If he tells them they have it, and where it is, they may give him $30,000/year as a worker bee. He says "Screw it. Why bother? If the mineral yield is expensive/poorer than anticipated, they'll be after my hide, and if it works, I get a paltry sum." I agree.
You are thinking in terms of the technology you "know", and not of the drive to develop a new technology when you throw out your "it'll never work because the numbers are too big" argument. The failure to develop profound new technologies is endemic to socialist economies, by the way, as they all devolve into bureaucracies, and there is no better way to smother growth than to have 10,000 GS-5's go after innovation because it challenges their place in the hierarchy of things...
Just as an example, assuming they set up the filter using a small portion of the Gulf Stream flow, your entire argument sequence falls apart. (I'm not saying that's the best plan -- wouldn't want to screw up the climate changes the Gulf Stream brings to northern Europe, for just one thing)