Really? I don't see a large market for seawater. The supply is immense, so there should be a demand created for it, especially since it is used as a raw material for fresh water, a product which is in high demand in many areas.
Er... Maybe you should think a little harder.
Nations have fought throughout history and millions died for seawater - access to seawater, specifically, because there is so much seawater that you can use it as ultra-cheap fast transportation. (Russia alone fought several wars just to get one warmwater port anywhere, salt or fresh.) Besides that, seawater is sold commercially for fish and animal keeping; deep seawater is sold as a dietary supplement (popular in Japan); and much more importantly:
since it is used as a raw material for fresh water, a product which is in high demand in many areas.
You even provided your own example! Desalinization plants are very expensive and produce expensive water... water that is feasible because seawater is free. Supply creates its own demand.
Real per capita income is simply total production divided by population. I doubt that real per-capita income has been increasing at anywhere near a constant power across history, rather than remaining roughly constant except during periods of technological development and/or social upheaval.
Oh for heaven's sake. Go look it up!
Why didn't anyone think to pump seawater into trucks and meet Russia's need for seawater? Because Russia didn't need seawater, it needed transportation links.
Desal plants typically don't have dirty water shipped to them, they are located where dirty water is. The limited resource consumed by their construction isn't seawater, it's location.
If sea levels rise, the added volume of seawater will not make desalination cheaper, just like a surplus of labor in the presence of labor price controls doesn't create a demand for labor at the controlled price.
Imagine you accept the main idea of cryonics, that if we freeze brains future technology is likely to be able to extract the encoded information and revive the person digitally. [1] While this currently costs about $120K/person, if we did it routinely to everyone it could probably get down below $1000/person. Which is interesting: the current cost of averting a death is around $1700, but someone who doesn't die of malaria is still going to die of old age, so you can't really say their death was "averted". While someone who is revived after being frozen wouldn't live eternally, they might get to experience thousands of years of subjective life. In terms of life-years, getting cryonics to be really cheap and paying for people to get it sounds like it beats GiveWell's top charities.
Aside from not agreeing that cryonics is likely to work, however, I don't think the value is actually all that high. A future world which would revive large numbers of people we freeze today would be massively different from the current world economically, but would still have constraints. There would be some number of digital people that could run simultaneously on whatever people-emulating hardware they have. Preserving additional 21st century minds would give future people the option to run revived people instead of new people, but I don't think it affects the overall number emulated.
[1] I don't think this is likely.
I also posted this on my blog.