Actually, it is meant to float: before the interior is completed, just at the limit of buoyancy so that it can be towed into the right position before it is sunk to its place. Buoyancy doesn't matter once it is at the bottom and covered over.

_{*Lunatic*, n.
One whose delusions are out of fashion.}

Ferrocement, and/or larger tube diameter for buoyancy, now that I don't see why it can't work. You should patent it :-)

_{*Lunatic*, n.
One whose delusions are out of fashion.}