If you don't know where the border is, stay very well away from it. Iraq and Iran have never agreed on a border. The usual rule is that sea borders are 12km out from the coast; but the place where this incident happened (whichever side you believe) is within 12km of both countries. So it's always going to be risky territory. In the event, they were taken in on suspicion of trespass, and released without charge. A propaganda victory for Tehran, egg on the face for London.