Have you seen the trailer for the new Robin Hood movie, the one that shows a ship with lowering bow like a WW2 landing craft? (See it here. The ship's near the end.)
My son's reaction said it all. "It's Saving Private Robin!"
I don't mind some anachronisms in books or movies because, for instance, we'd never be able to understand a word of dialogue if writers were strict, but that? Is a landing craft too far. I don't know that I'll be able to watch the movie, because that's the kind of thing that will drive me nuts.
Also, say what you will about King John (and note he was so unpopular, there has never been another king of England named John), at least the man actually lived in the country for more than a few months, unlike, say, his brother Richard.