Marnie had that Ursula Andress look
I noticed how much Tippi Hedren had the Ursula Andress look in this movie. Particularly when she was bleaching in her blonde look in one scene. She shakes her wet hair as she rises up to take in her fresh blonde look. I thought immediately of Ursula Andress rising up from the sea in 'Dr No.'
I think that Hitchcock would have been impressed By that 'Dr No' scene. He had Sean Connery in 'Marnie.' So perhaps he wanted an Ursula Andress look to go with for his leading lady. Perhaps Ursula would have been too busty. But, in any case, I don't think Tippi Hedren could have been beaten to play Marnie.