A thermal energy storage system includes a container positioned within a surrounding body of water and comprising a container wall. The wall has an interior surface exposed to and defining an internal volume of the container and has an exterior surface opposite the interior surface and exposed to the surrounding body of water. The internal volume is substantially full of water, and the container is configured to thermally separate water within the internal volume along the interior surface from water of the surrounding body of water along the exterior surface. A thermal source in thermal communication with the water within the internal volume is configured to transfer a thermal potential to the water within the internal volume.