Dredging is immoral - so incredibly destructive to the ecosystem. Do you harvest apples by bulldozing the orchard and sifting out the fruit? It's bad enough that so much of our farming is still based on tilling.

I don't really understand why nodule gathering isn't already done - just with some kind of robotic fingers or aimed suction devices. It's not as if nodules are hard to discriminate. Sure, there would be some interesting engineering challenges to operating equipment at scale in that environment, but it's undergrad-engineering-club-level, not rocket science...