I am just not sold. 100m/200m is nothing compared to 700m. Besides the mass increase needed for a concrete anything to survive that depth they want a turbine there too? Do you float these things when they need service? What if you can't? Like it fills up but cannot be pumped out? You are going to lay cable how far out into the ocean and across land, installation will be expensive and maintenance not much better.

there are just too many things that are expensive with this type of solution. Just like movies, when you thrown in scads of water things go wrong and they go expensive.

Just build carbon fiber spheres, bury them, and pump them full of air or other compressible.