My money is still on low-observable attack craft, or a high-low mix that deprioritizes submersibles. Iran has an impressive panopoly but also has casus belli to lie out their nose. 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 could definitely be wrong though, I don't have any insider info to work with here.