For backgrounds sake, let's say that there are large, and I mean large beings living underneath the icy surface of Europa. Now consider this excerpt from NASA's page on Europa:
One of the most important measurements made by the Galileo mission showed how Jupiter's magnetic field was disrupted in the space around Europa. The measurement strongly implied that a special type of magnetic field is being created (induced) within Europa by a deep layer of some electrically conductive fluid beneath the surface. Based on Europa's icy composition, scientists think the most likely material to create this magnetic signature is a global ocean of salty water.
Is it feasible/possible for a large, water-borne organism to harvest and convert the magnetic force generated by the moon/ocean itself into a sustainable source of energy?