C-Sub powered navy cannot fight a blue water or near blue water navy! This thing will lack power, propulsion and fuel to get in to the high seas, thats where most of the action is. And why wud its sonar signature be difficult to obtain? Sure it may not "glow" like a steel structure, but concretes do need steel rods, and I dont think it'll be very difficult to identify it. I can only remember "Red October". Other than that, I dont think any modern navy let alone USN needs to go near the shore to fight the enemy.
May be Ethiopia can use it against Ertriea.
