Just to add to malcolmgsw's comments.

The film has some elements of truth to it. The original destroyer concerned was USS Buchanan, renamed in the Royal Navy HMS Campbelltown. This ship did indeed carry out the raid on St Nazaire, in March 1942. The purpose was to prevent the use of the dock by the German battleship Tirpitz in case she should ever breakout into the Atlantic. It was the only dock outside Germany which could accommodate the ship.

One scene stands out for me in the film. It takes place in ex boxer Sid James's pub where some of the crew of our ship are relaxing. A crew of another ship starts to wind them up and this leads to a fight. However, we don't see this fight. As it begins, a bell goes off like the bell at the start of a boxing match. At the same time, the camera focuses on a photo of Sid in his boxing days, one of many which adorn the walls of his pub. Then as the fight ensues, the camera fixes on other boxing pictures from the wall as each blow is struck. Very enjoyable!