A couple of years ago the US Naval Research Laboratory announced that it was working on a method to break down seawater into carbon dioxide and hydrogen gases, then catalytically convert them into liquid petroleum fuels. It was hoped to develop a system that could eventually be used by US Navy vessels to derive their own fuel from the seawater in which they moved.
It looks like their research has borne fruit. Yahoo News reports:
The US Navy believes it has finally worked out the solution to a problem that has intrigued scientists for decades: how to take seawater and use it as fuel.
The development of a liquid hydrocarbon fuel is being hailed as "a game-changer" because it would significantly shorten the supply chain, a weak link that makes any force easier to attack.
. . .
The predicted cost of jet fuel using the technology is in the range of three to six dollars per gallon, say experts at the US Naval Research Laboratory, who have already flown a model airplane with fuel produced from seawater.
Dr Heather Willauer, an research chemist who has spent nearly a decade on the project, can hardly hide her enthusiasm.
"For the first time we've been able to develop a technology to get CO2 and hydrogen from seawater simultaneously, that's a big breakthrough," she said, adding that the fuel "doesn't look or smell very different."
Now that they have demonstrated it can work, the next step is to produce it in industrial quantities.
There's more at the link, and in a news release from USNRL.
This is obviously still some years - perhaps a decade or more - away from fleet-wide introduction; but it offers the possibility of virtual energy independence for conventionally-powered ships. They'll no longer have to rendezvous with tankers to refuel at sea, or return to harbor for the same purpose (although they'll still need to resupply with food, munitions, spares and other essentials). I've no doubt navies all over the world are watching developments intently - and some of them are doubtless already trying to figure out how to copy (or just plain steal) the new technology.