She sees a shadowy figure rise up out of the waves, the source of the singing. Despite it being bright out, she can't really see any of the figures features, like it's wrapped in a cloak of darkness. What she can see is that it's cradling a small figure in it's arms bridestyle.
They walk over to the rock and set the figure down on it. They look up at Rein, right at her, and while she can't see any other facial features, she can see the figure flash a sharp, wicked grin at her before it slinks off, back into the ocean.
