Providing the intelligence and coordination for Iranian proxies launching 100's of missile attacks on shipping, also operating in international waters, is plenty of justification. Iran has effectively shut down the Suez Canal adding massive costs to supply chains affecting prices all around the world. That also is an enormous consequence of proportionality.

Sure. But are we ready to get into a hot war with Iran over it? There's a reason these conflicts are proxy conflicts, and why the belligerents take great pains to keep them from boiling into direct hot wars between the main parties. If we got into an actual shooting war, it wouldn't just be the Red Sea but also the Gulf that got shut down - adding even more massive costs to supply chains all around the world. It is just far too important to the interests of the US (and nearly all of our allies) that the region remain somewhat stable that it's incredibly unlikely that we would initiate a hot war with Iran.

Iran knows this. So if we threaten to sink an Iranian-flagged vessel in international waters, the most likely response from them would be to ignore the threat and do whatever they want with the vessel. It's not a credible threat.