A lithium-water battery provides reliable power for long durations in the ocean at temperatures down to 0.degree. C. and pressures up to 680 atmospheres (10,000 psi) without the need for mechanical pumps or valves to admit reactant water from the ocean into the battery or to maintain a circulating electrolytic solution in the battery. The battery has a natural circulation, alkaline, aqueous electrolyte contained in the housing with a lithium anode and a cathode disposed in the electrolytic solution. A hydrophilic cation exchange membrane attached to the housing is disposed between the electrolytic solution and the ocean environment surrounding the battery for retaining the hydroxyl ions in the alkaline electrolytic solution while admitting water into the solution from the environment.Advantageously, a low power lithium-water battery can provide several watts at about 1.4-1.5 volts for up to a year or more anywhere in the ocean.