you can now extract (like mining) minerals from the ocean, sounds kind of dangerous for the ecosystem maybe? making it profitable to extract magnesium, lithium, salt... we can probably guess how this story goes.
i'm hoping it doesn't scale, honestly.
You're worried we might use all the salt in the sea for some kind of ... salt pyramids, send the water back out through sewers, and consequently leave the world's oceans diluted? That's about 1 followed by 21 zeroes, I think, in liters.
You're wildly underestimating the scale of the ocean. If we could extract all our necessary minerals from it rather than mining them that would alleviate a huge cause of environmental damage.