Would Iran be as likely to have hit one of these ships if the result was potentially a nuclear meltdown close to their shoreline? It would pretty radically change the costs to them for such an attack, wouldn't it?
Not every culture encourages the same principles or values as yours. I'd say they possibly would.
If you sink nuclear ship you won't get nuclear meltdown. You have enough thermal mass to cool down anything if you are underwater. Some level of loss of nuclear material would happen. But meltdown is not something that would happen.