it's pretty well authentic. Assembled in lines to wade out to any boat that came to their rescue they were bombed,strafed and shelled but reassembled again and again over days until they were taken off the beach. Waiting and expecting attack was their lot. A pretty good film but sad. http://www.slate.com/blogs/browbeat/2017/07/20/what_s_fact_and_what_s_fiction_in_dunkirk.html
It's authentic apart from the names of the soldiers - a bit of a sore point with some of the survivor's relatives.