Increasing global energy demand has driven the exploitation of oil reservoirs in deep oceans. The high risk of oil spill due to unexpected failures of undersea facilities urges the deployment of autonomous underwater vehicles (AUV) for frequent and thorough inspections in these extreme environments that humans cannot easily access. To meet the long endurance and occasionally high power requirements of the AUV, we propose a lithium-bromine/oxygen fuel cell with a protected lithium metal anode to provide high specific energy at either low-power mode with seawater (oxygen) or high-power mode with bromine catholytes. The proof-of-concept fuel cell with a flat catalyst-free graphite electrode can discharge at 3mW/cm² with seawater, and 9mW/cm² with dilute bromine catholytes. The fuel cell can also be recharged with LiBr catholytes efficiently to recover the lithium metal anode. Scanning electron microscopy images reveal that both the organic electrolye and the bromine electrolyte corrode the solid electrolyte plate quickly, leading to nanoporous pathways that can percolate through the plate, thus limiting the cell performance and lifetime. With improved solid electrolytes or membraneless flow designs, the dual-mode lithium-bromine/oxygen system could enable not only AUV but also land-based electric vehicles, by providing a critical high-power mode to high-energy-density (but otherwise low-power) lithium-air batteries.