Underwater benthic microbial fuel cell (BMFC) systems were studied as a long-term sustainable power source for low-energy aquatic instruments (e.g. sensors). Different electrode materials and a power management system (PMS) arrangement were investigated in the lab-scale BMFCs over 12 weeks. An activated carbon (AC) cathode without catalyst layer (CL) was examined as a low-cost cathode material, and the results showed that AC cathode with a diameter of 5 cm and 20% weight polytetrafluoroethylene (PTFE) was the most effective. Granular Activated Carbon (GAG) column was used as a cost-effective anode for biofilm growth. The efficiency and durability of GAG was compared with other anodes with high surface area (e.g. carbon fiber brush (CB) and activated carbon nanofibers (ACNE)). Although, CB and ACNF outperformed GAC, and could be used as the model anodes in underwater BMFCs, high mechanic strength and durability of GAG are advantageous for long-term operations in benthic applications. Evaluation of a caged BMFC system equipped with multiple pairs of AC cathode and GAC anode showed the peak power and current densities of 0.56 W/m(2) and 0.25 A/m(2) respectively. A PMS was developed and coupled with batch mode BMFC system, to boost the output voltage to 3.3 V necessary for the working load. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.