Disagree with the premise that it couldn't be a shaped charge.
IMPORTANT DISCLOSURE: I am not directly aware of the TTPs and technologies of military underwater demolitions. My analysis is purely speculative based on my understanding of the situation and previous study and experience with unclassified information.
Firstly, hydrostatic shock and pressure can be used to redirect, and this make more efficient, a shaped charge's penetration of a harden surface. It is an old breacher's trick that you can breach a steel door with nothing more than det cord and an IV bag. The nature of underwater demolitions must change the dynamic.
Secondly, we dealt with EFPs (Explosively Formed Projectiles) where a high-heat, self-sustained charge would melt a disc of metallic material into a molten penetrator that would draw the rest of the charge into the cavity. It was capable of breaching tank side armor. Concrete has a very low tensile strength, and doesn't hold up well to explosive pressure. It might also ignite the gas depending on the oxidation and fuel mix (and therefore cause a larger secondary/tertiary explosion).
Thirdly, the depth of the attacks is not terribly deep. This opens up the possibility of less capable dive and UUVs (not necessarily military).
Conclusion: everybody is, yet again, jumping to conclusions that they cannot justify or prove. I'm not saying it wasn't us. I'm just saying no one knows. Except for the strategic intelligence capabilities of nation states, no one is even sure of who was sailing in the area.
My strongest indicator that it was the US is how incredibly terrible the decision making track record is (intentional or not) of the Biden administration. There are plenty of people stupid and capable enough to have done this, but here's the kicker:
It doesn't matter. Everyone, especially the people with the militaries and nukes, has likely already decided who they will blame AND what they're going to do about it.
I don't know about you all, but I'm going to eat more steak in the immediate future.