In this work, a high-specific-energy magnesium/water battery (Mg/H2O battery) combining Mg oxidation with hydrogen evolution reaction (HER) is developed for full-depth ocean application. With the optimized platinum loading associated with moderate Ni(OH)(2) on nickel foam, the performance can be increased obviously. Coupled with AZ91 Mg alloy anode, the specific energy of Mg/H2O battery reaches a dramatic value of 1003 W h kg(-1). Moreover, ultrahigh pressure of 1100 bar in the deep-sea simulated condition brings negligible effects on the performance of battery and electrodes. The influence of low temperature can be reduced to an acceptable level under the application at low discharge current density. Stable discharge of the battery for 100 days is also obtained. This work provides a new idea to supply power source with high specific energy, excellent environment adaptability, good safety and low cost for underwater application. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.