I just haven’t been able to find a technical discussion that says why ordinary ferrocement cannot be x-rayed. It seems like it can be, it’s just expensive. I see lots of discussion on X-ray vs ground penetrating radar (GPR) and even mobile imaging using sonar and IR, with some discussion of resolution of GPR or other imaging possibly not being fine enough to discover flaws other than voids.
I agree that geopolymer and basalt reinforcement is clearly the way to go, especially since fabric-like mesh allows for even thinner and more consistent hulls. I would plan on a fairly substantial thickness, given that it is not intended to be a racing yacht or make any great speed, rather being built for decades of low or no maintnenance rather than racing performance. Given that basalt and geopolymer cement probably have similar density and radar profile, I’m not sure how useful X-rays would be other than finding fissure, voids, etc.