I also couldn't see any obvious problems.
I took the freedom, so to say, to render the image at 800x500, quality 0.7 AA 8 (for the sea) and GI 2/2. During rendering the memory usage was quite high for such a scene. I guess due to quite high quality settings in your clouds and atmo (I left them untouched). Rendering at quality 0.8 probably increases memory usage even more.
So my thought is that it is a memory problem, but of course I can't be sure.
By the way, it is a very nice image
I'd reduce the roughness of the water because there are some awkward looking waves in it (middel of the image at the horizon for example).
Martin