Maybe it's because I'm not American, nor from any of the English speaking countries, but I felt that the treatment of the last third was not very favorable to Americans. Of course Columbia Pictures is selling the film as a patriotic one, but still there are quite a few negative depiction of US army. SEAL agents look so startlingly like thugs, their instructions to Phillips are thin, they are lying to both pirates and Phillips after the talk goes sour.
So, while we (and I) do know that the ending, and the suspence does not come from that; instead it's about how far Phillips's positivity can fall, how far he will be isolated, etc.. In that respect the later half was full of desperation, and that was what my friend who watched the movie together agreed with me a lot. (She is a great filmgoer as well)
BTW, at the end of this talk we were both wondering if others - especially Americans - would detect the same thing on the depiction of marines. Obviously you are British, but I wonder what you think about this.