First off, I know nothing about under-water warfare, I have no information other than layman’s information on remote sensing.
But, if I were in that area, there are some key points I would focus on. First off, Submarines are made out of metal, random rocks on the bottom of the sea are not.
Metal has properties that granite, limestone and other assorted rocks do not. I’d wager that if one was interested in finding a couple hundred thousand pounds of refined metal, there are ways of doing that.
Many ships, as I understand, are degaussed, so you can't depend on a static, localized deviation of the magnetic field. You could detect metal from a distance by using its ferromagnetic influence on an inductor, just like it's done in a common metal detector. However the distance that such a metal detector covers is measured in inches. You can increase the sensitivity by building a coil as large as a square mile, but then you wouldn't know where within that square mile the metal is located; and a motion of a wristwatch on a sailor's wrist will cause a reading 100x larger than a sub at the bottom.
On top of that, the area is an active sailing zone for last five or so thousand years. You probably can find a lot of metal and sound reflections on the bottom, perhaps even in larger pieces than a submarine. A search there would be an exercise in marine archaeology.
Other posters mention that these subs can't regenerate oxygen. But why not? There are many possible technologies, and a sub can be always fitted with this or that machine instead of something else that they can sacrifice. If they keep the crew to a bare minimum (this is not a long sea voyage, after all, but just a couple of hours from the shore) they can have plenty of air. If the sub is largely powered down its batteries will last a long time. If I were in charge on Iranian side I would even consider a power cable to the shore. Then they can stay at the bottom as much as they want since if you have power then oxygen can be produced from seawater. They don't even need to run audio equipment; all they need is to have a VLF radio on.
With regard to tankers' captains braving torpedoes and mines, they themselves may be insane enough to try, but the owners of those tankers will probably say "no." Tankers are expensive. Besides, if the sea route is blocked the oil price goes through the roof, and owners of those tankers will be earning more money on the rest of their fleet. I don't necessarily see a good reason for the tankers to try to go through.