My experiences with the ferry to Iran:

Oasis is the company that operates the major ferry from Sharjah (the most "Persian" of the Emirates), UAE, to various ports in Iran. If you contact the ferry companies in Iran you will find they all are agents for this same ferry.

It is possible to hitch a ride on a freight ship, but you need to take the captain of the ship to Immigration and Customs to complete the paperwork. For all but the smallest ships this is not worth their while. Some ships will point you to their agents which will usually charge a considerable sum to do the same thing. Most larger ships are simply not interested.

UAE or Omani Immigration authorities do not allow foreigners to travel to Iran on small dhows from their ports - these captains have special travel permits allowing them to come and go as they please and are not sufficiently regulated to satisfy Immigration officials that you will arrive safely. I've found dhow captains are more than willing to oblige, shame about Immigration.

One time I tried to travel to Iran the maritime border was closed (late March/early April). I heard all kinds of excuses for the cancellation of the ferry but the most likely reason was the Iranian navy's exercises at that time. Be warned, these exercises occur often - fisherman and small traders are routinely picked up and held indefinitely for encroaching - and the ferry is often cancelled.

In summary, although there are likely to be exceptions, generally speaking there is no maritime transport between Oman and Iran, and only one ferry between UAE and Iran. This one ferry is regularly cancelled at the whim of the Iranian Authorities and is often fully booked. Book early, try not to conicide with Iranian holiday periods, and hope the Iranian gunboats aren't doing their stuff.