Oil Tankers or Aircraft carriers. The problem is I imagine that a sub that carries these is being manned by the IRGC Navy whose sailors don’t care about their own lives, which would make them more of a threat.
I’m sorry, I just don’t see Iranian subs as a risk. They have a total of three Russian-built kilos and a handful of knock-offs of a North Korean mini-sub. None of them is ever going to get within shooting range of a USN ship.
OTOH, there are places in the Gulf where ships pretty much have to operate within range of shore. Or, for that matter, within range of commercial vessels.
I can see a couple of scenarios where they could use this in a surprise attack. I can conceive of no way they could use this in a sustained campaign. So, at best, they could damage one US destroyer and lose all of their naval facilities in exchange. Which even they aren’t stupid enough to do.
What’s going on is simple - they bought an old Soviet-era weapon from the Russians, who’d found they had no use for it, and fired it off in an effort for domestic propaganda purposes. The Russians, of course, thought it’d be a fine idea because it would keep tensions up, and hence keep oil prices high.