My back has also that very little gap between the back and the frame. Do far so good with Waterproofing. I always go on lake to swim. Just avoid seawater and you'll be fine
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Yes, I have just called Sony Mobile Support and service for Slovenia and they said it's totally normal like this and it's just a cosmetic "issue" and most of the devices have the same as they even checked those that they had there. And even IF something happens to the phone Sony will cover it. No worries