The U.S. Naval Research Laboratory has been releasing information over the past few years on a new method of creating liquid fuel from sea water. It sounds incredible, but the process is straightforward chemistry, using an "Electrolytic Cation Exchange Module (E-CEM)", they are able to extract carbon-dioxide and hydrogen gas with a 92 percent efficiency. Prior sequestration techniques were less efficient, and produced high levels of unwanted methane. The latest system reduces the amount of methane and produces much higher levels of unsaturated hydrocarbons which can be converted to liquid fuel using a catalyst reactor system. This second step is key as it converts molecules with low molecular weight, in this case the hydrogen gas, to one of higher molecular weight, capable of being converted into liquid fuel.
Does this mean we will see a fleet of ships running on seawater in the future? Not likely. The NRL is not forthcoming on the amount of energy needed to convert seawater to fuel or the efficiency of the process, but the purpose of the system is not to provide fuel for ships, but fuel for aircraft. Most aircraft carriers are powered by a nuclear reactor, providing a nearly inexhaustible source of energy, but the aircraft they carry still run on JP5 jet fuel. Fuel must be stored and replenished regularly, limiting the operating range of the carrier without support. This technology provides a means of producing jet fuel on demand at sea, a strategic advantage that also reduces military dependence on foreign oil.