The problem with a lot of ocean "exploitation" is that it's like mining a human for calcium. Yes, a human body contains ~1kg of calcium, and you could take it out, but it's already *doing* quite a lot of useful things, and the human has far more potential to help you if you leave the calcium alone.
@helenczerski I suspect most people don’t really appreciate how dilute ocean resources are, and how much collateral destruction their mining would involve.
Take your analogy, you can get the kg of calcium by destroying the surrounding 100kg of person.
For seabed deposits, what if you have to ruin 10,000 kg of seabed, 100,000 kg of seawater, and a million living things to get your kg of manganese?