Uh-oh
They were in the Northern Gulf, near the Shatt al-Arab waterway which is a highly contentious dividing line between Iraq and Iran. The rules of territorial jurisdiction in the Persian Gulf are ambiguous--Iran is claiming that the Marines were in Iranian territory at the time of their capture, while the UK and US are insisting that they were in Iraqi waters and were 'forcibly escorted' into Iranian waters. This isn't the first time this has happened--8 UK Marines were kidnapped in 2004 and held for three days, but this is, to put it mildly, not the best time for another international incident with Iran. This could blow over, or it could not. Please, guys, let's be reasonable about this.
[UPDATE] Well, this is is just delightful. The foreign embassies in Tehran are updating their escape plans in preparation for a possible US/Israeli strike.