The continental shelf averages 300 to 600 feet before it drops off into the deep ocean. But the question still remains how to you extact minerals even from that depth.
The technology doesn't exist and it would only be feasible if all the earths land based minererals were exhausted. We are along way off from shortages of any mineral or metal that I am aware of.
To put into perspective. The economic and technological challenges of underwater mining are on the same order of magnitude as mining on the Moon which I find to be an equally zany idea.
I think first come, first served has worked well to date.
Hydrocarbons