Fair point about Boromir on the boat.
I maintain that the story throughout the first film was leading towards Boromir's turn, though. Even if it was only for a few crucial moments. The events led to Boromir's turning bad at the end. It was a twist. So what if the heel turn didn't last until the end of his career?: look at Ric Flair.
The heel can regret his actions later when he turns back face, just like Boromir did. If you want to get really picky I could change it to say that Boromir displayed heel mannerisms, even though he didn't fully turn heel.
*Sincere apologies to all non-wrestling fans*