> Most of those unmanned narcosubs are cut-down speedboats hulls, to my knowledge.

Some are now fully submersible: https://insightcrime.org/news/under-radar-what-hundreds-ofna...

I think it is indeed more likely that they used a low-profile boat, but I won't discount a full submersible. Or maybe a combination: a low-profile boat that uses a regular outboard gas engine to get close to the target, and then dives and attacks like a torpedo.

> If Iran does have fully submersable UUVs, I'd expect them to be saved for a direct confrontation with the US Navy, not tankers.

I don't think they can do serious damage to large US Navy vessels.