Dreadful. I would have walked out except for the fact that my son took me.
A significant sequence involves the civilian boat captain rescuing the pilot of a ditched Spitfire from drowning. Why was he drowning? Because he ditched with the canopy closed!
However, the shots of the Spitfires and the Blenheim were nice.
All-in-all, it made me decide to watch the original movie again. I was living in Ramsgate when they made that film and they used the harbour entrance for some shots, complete with sand bags, barbed wire and AA guns.
https://en.wikipedia.org/wiki/Dunkirk_(1958_film)
Starring John Mills, Richard Attenborough and Bernard Lee
The film was the second most popular production at the British box office in 1958. According to MGM records it earned only $310,000 in the U.S.A. and Canada, but $1,750,000 elsewhere, resulting in a profit of $371,000.