Lot easier to create a skeleton in the shape of a cylinder(cassion[1]) as you said, versus a sphere.
[1] http://www.thebluebook.com/inc/img/qp/75476/rebar-cages-for-...
The reinforcement cage in concrete weight coating used on subsea pipelines is relatively ordinary steel. The steel is protected from corrosion by the concrete itself and a system of sacrificial anodes that are easy to survey and replace. (I help design pipelines)
I guess that the steel is protected by something else because, as the pre-1970 builders ignored and as it dawned on everyone else 20 years ago, concrete is not waterproof.
Ancient Roman concrete is.