I'd say it's Safe and not even a border case. (Although it might qualify for the new "Risque" rating, if that happens.) We're not seeing any more boob than if she'd been wearing a typical bikini, she doesn't look aroused/naughty and her breasts are not the focus of the image.
Let me reiterate my standpoint from topic #12067: Despite its name, the Safe rating is not about a post being "safe" for anything. What it does mean is simply that eroticism, gentle or blunt, isn't the post's primary theme. To "err on the side of qaution" (or caution) is to miss the point.