According to WIKI they currently do have operational full size Submarines and Frigates. They don't just have the little patrol boats and gunboats and corvettes we see on the news harassing Western countries navies. I wonder why they don't attempt to come at the Western navies with those Frigates and Submarines? It's like bringing a knife to a gun fight.
Do you really think the Iranians are trying to win a naval engagement using those small boats?
The harrasment is for propaganda and/or provocation.
They can record footage of 'challenging the Great Satan's' navy, edited and narrated to suit their purpose.
If they do manage to sufficiently provoke the U.S. Navy, they may try to use that as justification to bring their land based missiles to bear. It would almost certainly prove to be an unwise decision, but that doesn't necessrily mean they won't make it. If things get bad enough domestically for the Mullahs, they may start viewing that idea as feasible. Likewise is things in the U.S. look bad enough to the Mullahs to the point that they think they can get away with it.