It is amusing to read the reviews by the film critics.
They pan it as not being solid, not up to Cary Grant's standards, empty, vapid etc.
Forget the critics:
It's a great light story set in WW2 about a carefree freeloader who is coerced into being a coast watcher by Trevor Howard's character, A Royal Navy Officer. Trevor Howards is great, Cary Grant is just plain funny as a batchelor who must rescue Leslie Caron and her girls.
This film works because it isnt presumptuous or glamourous or serios: it's a comedic battle between Grant's character and Howard's and Leslie Caron.
It's witty and fun and that's all that is really important