Cliff Robertson portrays John F. Kennedy during his late-20s during World War II; Robertson was 40 years old when this film was released.
When President John F. Kennedy saw early footage of parts of the film, his only complaint was that Cliff Robertson was parting his hair on the right, while JFK parted his hair on the left. Robertson dutifully parted his hair on the left for the film.
It was decided and approved by President John F. Kennedy that Cliff Robertson speak in his natural voice and not try to imitate JFK's Boston accent.
The real PT 109 had been engaged in continual combat for five months when John F. Kennedy took command; it was not the derelict, inoperative hulk originally depicted.