Abstract: Technologies such as stealth buoys and underwater gliders need to modify their own buoyancy in order to operate. Strategies such as pumping fluid are typically used to change the device's net volume. This in turn requires a mechanically sophisticated apparatus, increasing the cost of the vehicle while diminishing its reliability. The concept of a buoyancy engine that exploits the enormous volume and pressure changes accompanying the reversible electrochemical interconversion of water to hydrogen and oxygen gases is applied to stealth buoys and underwater gliders.