Superficially it's a war drama of the tiny motor torpedo boats which did much unsung work in WW2, but the naval battles merely provide an exciting story in which an even more special romantic drama is wrapped up. The difficult love triangle involves the stuffy, awkward Brit and his unhappy wife, complicated by the arrival of the flamboyant American. The three play their parts beautifully as the tension rises; husband suspects wife, she is agonisingly torn between the two, lover tries to win her over.
Additionally there is an element of professional respect between the officers, which only serves to frustrate their base instincts, as the competent yet uncertain Brit tries to hold onto both a command and a wife he knows he doesn't deserve, and the daring yet sensitive American (a divorcée?) starts to see some semblance of bravery fighting to break out from the Brit's inhibited facade. Very like real life, the path of love does not run true, and the result is thus more realistic than one would expect from a movie. The madcap plans to outwit the enemy mimic perhaps, the deceptions played out in the romance.
This may not be a totally memorable film, but it has some finely understated beauty, which quietly avoids using hackneyed stereotypes, and mawkish efforts at pathos.