If your machine can handle it, throw in a little volumetric light behind her head over the pool area, since if you notice the light intensity on her face and dress, the darkness in the pool doesn't seem to be in the same physical space. Adding some volumetrics may indicate a cloud moved overhead and is cutting out some of that intense direct sunlight on her face and dress.
basically, drop the entire scene in the HDRI and rotate it till the light looks decent. You can add filler lights to brighten dark spots. If you can't find a night scene that works, take a day scene and dim it a little then add a "little" blue to it under the Environment or tone map I can't remember but it will be obvious when you try it.
Ill add that good textures are important as well. A well-lit cowpie is still a cowpie. There's a reason why most models and stars are beautiful people. It's rare to see photographers drool over ugly subjects unless something about there "look" is intriguing. Like an old man with shoe leather skin and lots of wrinkles.
If youre using an actual setup and not just the HDRI you might not need a large HDRI. something like 4k might be fine. If youre using only/mostly the HDRI youll probably want 8K or more. https://www.youtube.com/results?search_query=hdri+Daz3d