What was this movie about?

It may very well describe life on the sea but I couldn't tell what the movie was about. I found the movie boring. It had some great individual performances as well as some good scenes but it didn't focus much on the spy aspect of the movie.

I am writing this review with 10 minutes left. I reread the synopsis and I thought the focus was on worry about a possible spy on board. I come to find out that is only a minor minor part. I wonder where my head is sometimes.

I know it was a 4 act play weaved together but I still would like to know what the movie was about from someone who watched the movie.