Yes, Iran has already hit several merchant vessels. Their ability to do that occasionally is not in doubt. It's mostly a question of economics. The ship owners and insurers have to decide whether it's worth the risk to run their cargoes through. This has all happened before with the 1980s "Tanker War" between Iraq and Iran: despite some losses the traffic never completely stopped.

And large merchant ships, especially crude oil tankers, and quite tough to sink. When they take a hit it usually just causes some damage.