Firstly, I know practically nothing about Daz - but since nobody has answered, let me see if my complete lack of knowledge throws something up.
I assume the "spot"/"shadow" you're talking about is this bit as highlighted.
Except, from my point of view - that's a perfectly natural shadow considering the light source.
Correct me if I'm wrong here, but based on the shadows on wall from her arms - there is a single strong light source directly in front but slightly below the girl's waist. If I had to guess I would actually say it's below the surface of the water.
So that shadow is the result of that light being blocked by her leg shown on the left side of the image. And being so close to the light source, the shadow is much sharper and distinct.
The only two ways I could imagine
not casting a shadow like that in the real world would be:
Raise the angle of the spotlight so that none of the resulting light hits below her vagina level. Maybe even lower the light's position as the angle is increased to balance things out a bit. Though from an immersion breaking point of view - wouldn't she be electrocuted by the spotlight in the water?
-or-
Introduce a second much dimmer light source that shines directly down and to the left into her crotch, probably positioned on the right side of the image; level with her breasts and only slightly in front of her body to illuminate that area. It would need to be weak and targeted enough to only light that area between her legs without affecting the composition of the rest of the image.
I'm applying my view of how light works in the real world for this - so it maybe completely the wrong answer for Daz. But it feels like if I were trying to light this as you seem to be aiming for in the real world... I might be trying to use a horizontal facing spotlight to light her upper body and a downward facing vertical weaker spotlight to light her lower body. Effectively a hybrid of my two answers.
My first reaction was to put a tiny (soft) light ball just fractionally above her legs at crotch level to light that area. But the "how the f**k would you do that in real life?" kicked in and I tried to approach it as if I physically had to set the lights up around the pool.
Dunno if that helps... but I figure it can't hurt either.
Edit: From a weird "only on a computer" point of view... is it possible to change a parameter on her leg/lower body to NOT cast a shadow? It'd be impossible in the real world, but maybe it's the "Daz" answer you're looking for.
