Sea Like a Mirror

I don't know where the question setter got his or her information on the Beaufort Scale, but the phrase "like a mirror" doesn't appear in the Met Office's table – it uses the word "glassy".

No real problem with this – just that the question purports to use an official description, when it would appear that it doesn't.

Any expression that included the word "calm" – for example, "flat calm" – was to be accepted as a correct answer.