As you point out, that causes corrosion, using up the metals (well, using up their galvanic potential energy anyways). These nodules are A) extremely old and B)

As you point out, that causes corrosion, using up the metals (well, using up their galvanic potential energy anyways). These nodules are A) extremely old and B) extremely slow growing/generating. If that were the source of the electric fields, it couldn't be sustained long enough to be a stable source and it would result in the nodules corroding away (or at least one of the metals corroding until there was no longer electrochemical potential, which would end the effect). In other words, for that to be the source, then that would imply that something about our understanding of how these nodules form would be off by many orders of magnitude. They would have to be forming so quickly, that the concerns about using them up would almost go away (assuming that harvesting didn't disrupt the process of formation that is). Part of the reason why there is concern here is that these nodules (as we currently understand it) form so slowly (literally millions of years), that if we harvest them, then they are, for all intents and purposes, gone forever. So if they do have important ecological functions, then those functions are completely wiped out. But if they are actually forming on the order years (and I can't imagine that it could possibly be any slower than that for galvanic/corrosive processes to be the answer, and in fact probably faster), then that concern goes away. Harvest one section of a field, go away for a decade, and it's restored.