Kane's Sketchbook.

Have you ever wondered what Kane’s sketchbook/log looks like? 

Why does Castalia want to look at it all the time?

I imagine his log to be full of drawings and notes. At one point he is absolutely aware of the fact that she is reading it and then she even starts writing little messages for him.

So maybe the drawings he was showing her on the island, after she took a bath, were something like this.
Hence the line: these are a bit....
And his answer: nude is not naked.