The capture of British Marines by Iran has happened before, then on the Shatt-al-Arab waterway. It will doubtless be used by those seeking to bang the war drum against Iran, though I imagine it will be fairly quickly resolved.
Before people get too carried away, the following is worth bearing in mind. I write as a former Head of the Maritime Section of the Foreign and Commonwealth Office.
The Iranians claimed the British soldiers had strayed into Iranian territorial waters. If they had, then the Iranians had every right to detain them for questioning.
The difficulty is that the maritime delimitation in the North West of the Persian Gulf, between Iraq, Kuwait and Iran, has never been resolved. It is not therefore a question of just checking your GPS to see where you are. This is a perfectly legitimate dispute, in which nobody is particularly at fault. Lateral maritime boundaries from a coastal border point are intensely complicated things, especially where islands and coastal banks become a factor.
Disputes are not unusual. I was personally heavily involved in negotiating British maritime boundaries with Ireland, France and Denmark just ten years ago, and not all our own boundaries are resolved even now. There is nothing outlandish about Iranian claims, and we have no right in law to be boarding Iranian or other shipping in what may well be Iranian waters.
The UN Convention on the Law of The Sea carries a heavy presumption on the right of commercial vessels to “innocent passage”, especially through straits like Hormuz and in both territorial and international waters. You probably won’t read this elsewhere in these jingoistic times but, in international law, we are very probably in the wrong. As long as the Iranians neither mistreat our Marines nor wilfully detain them too long, they have the right.