1. cement is actually rather light, c. 1.5 t/m³;
   
2. concrete is more dense, up to 2.5 t/m³;
   
3. however, methinks the buoyant tunnel would be made of steel, which is much more dense than concrete: 7.8 t/m³.

Or are we talking about the tube itself?  I was considering the tube to be made of steel alone, but now you mention it, a ferrocement design would be much lighter (steel has a density of about 8).  There are many ferrocement boats out there and ferrocement is lighter than concrete.