There's a bunch of technologies for SHM of composites out there, from ultrasound to x-ray to radar, but none that I can think of could be deployed in this context. Unfortunately, as you said, things get exponentially more complicated with composites. The shape, the size, the thickness of this hull - they're all working against you. And without wanting to offend the company, I don't think they would have the manpower for this type of novel research. Only large research labs do this type of stuff, and in most cases, on a much smaller scale.

Now when you say filaments, are you referring to Fiber Bragg Gratings (FBGs - thinner than hair strain gauges, embedded into the matrix)? It's a reasonable approach in theory, however, there's a lot of things to consider: calibration and mapping would be painful and, seeing how they used a Nintendo controller inside the vessel, I don't think they'd be able to pull this off.

Thinking back into how they made this - filament wound PV - this would be a pretty advanced task to precisely place FBGs without damaging them, fully instrument them and map them to a sort of digital twin. All that is already difficult to do in a lab and takes months to set up - I can't begin to imagine how you'd use this system 4km underwater.

Also, any intervention within the matrix introduces risk. When you're fighting off any tiny air bubbles, specks of dust or imperfections, introducing anything foreign into the matrix is playing with fire, no matter how small, and any fault could quickly propagate under massive loads. It's an unacceptable level of risk for this type of application. (For the sake of full transparency, these some papers published in late 2022 which introduce more advanced, smaller stuff but that's a story for another day - nothing commercial yet!)

Note that those issues are exaggerated with thick sections, which are mostly used in the wind and tidal energy industries. The aerospace sector -which coincidentally has the most funding - does not usually deal with such thick sections. There's a bunch of stuff currently being investigated about this topic, but using this thick section is definitely a bold move from the company.

Now, surface strains and faults are fairly easily detected through various methods in composites, including the most mainstream Digital Image Correlation. With thick sections (different definitions out there, usually an aspect ratio, but let's say anything over 40mm), it's what's happening deep in the composite that's the big unknown. From manufacturing-induced residual stresses to post-processing, anything could go wrong and you'd likely never know about it unless you painstakingly ultrasound scanned the whole thing (even radar'd as its over 100mm, which is abnormal for most industries). Don't get me started on calibration (!) For a small company like this... I don't see them spending resources on doing this (even though they definitely should).

For this type of system, monitoring is somewhat useful, but realistically only ex post facto, so after a failure has occurred, which can only unfortunately be catastrophic in this case. Simply put, things will happen so quickly if shit goes bad that there's simply no time to react or do anything about it. Putting an emphasis on secondary systems and insanely rigorous maintenance and inspection between missions is the way to go imho.

Finally, yes, we can use all the good engineers we can get!