Energy harvesting is widely used in terrestrial and aerial sensor applications but is conspicuously absent in the marine environment despite several possible harvesting modalities and numerous applications. One such energy harvesting modality is to use magnetohydrodynamic (MHD) power generators to directly produce electricity from flowing seawater. Fundamentally, MHD generators convert the kinetic energy of a conductive fluid directly into electricity by separating charged particles, thereby generating an electric field transverse to the direction of fluid flow and the magnetic field. The electric field is then accessed with an external circuit to provide power to a load. Since the power output from an MHD generator is linearly related to the conductivity of the flowing fluid and to the square of both the magnetic field strength and the fluid velocity, strong magnets and high fluid velocity are desirable. Thus, there are a myriad of possible MHD generator configurations available to maximize power output under various conditions and constraints. These include configurations of permanent magnets that offer localized high magnetic fields or geometries of the fluid duct that can be used to increase the fluid velocity through the magnetic field. One novel application for MHD generators is to power sensors and bio-loggers used in marine animal telemetry. The animal sensors are designed to take time-series measurements and store the data on the logger for transmission to satellite networks or human retrieval. These sensors and loggers are often battery-limited which constrains either the data fidelity or the longevity, or both. An MHD generator attached to a marine animal can help to supplement some of the sensor or bio-logger power requirements, thereby increasing sensor lifetimes and data fidelity. Thus, MHD generators will enable new research in the marine sciences, climatology, and biology, among others. The MHD generator can be positioned above the fluid boundary-layer so that the fluid flow around the animal is channeled through the MHD generator, producing electricity. In this work, we will develop some of the fundamental equations that describe the physics of an MHD generator and use them to make estimates of the potential power outputs that could be expected from various marine animals. We will also investigate several electrical configurations of the MHD to determine the most suitable MHD generator for different flow regimes. Initial studies suggest that MHD generators are viable power sources in the marine environment and can easily supplement the entire energy budget of a bio-logger under certain conditions.