Everything that's on the frame; the lighting, the costume the actors wear, their facial expression etc.
low lighting and darkness suggests they aren't safe in a source of light which in this scene is the fire, instead the dark seems to isolate them and show that they are vulnerable as the couple sit unincluded from the group and out of the way. The fire giving out light suggests the rest of the group are out of danger, however the couple are videoed as they move further away and into the darkness out at sea.
the sound before the girl gets into the water starts of loud with music playing
but as she moves away the sounds all become natural such as the waves of the se and their foot steps