Well what will happen is that US courts will declare it illegal, but after attention spans have moved on, so the end result will be that Iran just wins in the end, and oil gets more expensive.

This specific part of international law has been almost universally observed for many decades. It actually matters.

The problem is that the US extorting protection money from merchant ships for passing in an otherwise free international waterway makes the US government look like a mafia state. Which it may very well be, although it tried to avoid making it this clear before.

Everybody benefits from the free trade system working - and that depends on international freedom of navigation law being respected. Also Trump voting Republicans, and Republican Congressmen, benefit.

In a sane world, US voters would stop Trump. In their own rational self interest.

It’s fully within the territorial waters of Iran or Oman. I’m not an international law expert but it seems that it’s a practiced convention not a legally enforceable rule.

The right of "innocent passage" in a country's territorial waters is guaranteed by the UNCLOS. It's a legally binding rule since both Iran and the USA have signed the convention.

But both have probably also violated a dozen of international conventions since the start of the war, so that's mostly besides the point.

not a legally enforceable rule.

Anything is enforceable with enough bombs, if there is a will. Any previous US President, Democrat or Republican, would have enforced it no question.