This reminds me of another "pumped storage" type solution, but using rail cars of basically rocks. This of course has the advantage of working in areas where there's lots of solar and no water, like the desert.
A dude on the Danish engineering society worked through the math, and indeed, rail cars don't seem feasible. Just too expensive.
I don't have too high hopes for the sphere idea either - operating turbines in seawater 700 m down seems too complicated compared to transporting the energy to land and storing it there, somehow.
http://www.aresnorthamerica.com/grid-scale-energy-storage