Not an infinite submarine engine running on seawater. That was my brain getting carried away from just scanning the title. First,carefully read the article and then rush to conclusions. The article in question (DOI) explains how seawater can work as the one end of a galvanic cell, not for use in submarines but in a tiny explorer in a bot swarm, or so I imagine. Of course you still have to carry with you a stick of magnesium as the other end so it is not like the bot runs on seawater alone. It is different from a salt water battery where sea water is the electrolyte at most. A consortium of Chinese researchers with Zhang as first author are quick to point out that the concept is not new (2018 example of cathodic seawater here) but their twist has something to do with high energy density output vs high energy power. A difference they like to compare to how cellular respiration works: in aerobic respiration in a series of redox reactions the ultimate electron acceptor is oxygen, in Anaerobic respiration the final acceptor is a sulphate, nitrite ion or sulfur with a lower reduction potential and less energetic.
In the experimental setup the anode (electron supplier) is the magnesium stick and the cathode is Prussian blue, ordinarily an iron cyanide pigment. It acts as a coordination cage that fills itself up with sodium ions when immersed in seawater. The authors do not tire to mention that a seawater in question was sampled from the East China Sea but that should not be relevant. In the high energy mode (aerobic) a low power density resulted in a high voltage with electron acceptor dissolved oxygen - higher in energy but with limit supply. Increasing the power density made the voltage drop marking a switch to high power mode (anaerobic) with as electron acceptors the sodium ions which are abundantly available. In the high power mode sodium is incorporated in the Prussian framework with reduction of Fe(III) to Fe(II).