Well I thought Master and Commander was a bit overrated when it was released, because it was hailed for many wrong reasons imo. But that's usually what hype and the propaganda machinery of Hollywood do to films. 
But after the hype died down, I think it got underrated and remains at this.
I'd probably agree with Trevor. This is also the best film at sea I have seen. And contrary to wpqx, I'm a sucker for movies at sea 
I also love this kind of movies where nothing much happens plot-wise and you get to observe the interactions of people. A bit like Rohmer.