I think the issue is there’s a disconnect between her upper torso and her pelvis. This is exacerbated by the line of her shoulders being the same as her pelvis. You just need to spend a little more time at the beginning to lock in the pose before you start detailing the image.
yeah the pose thing tracks, though maybe the flatness is more the ribcage. it reads like one card shape under the clothes. i draw the ribcage as a rounded box and tilt it away from the hips, then let cloth wrap over that. helps even before detailing
locking the pose early makes sense. idk if the shoulder line is the whole thing though, the water hides where her hips sit so the middle floats. maybe mark the waterline across her body first and let the torso lean into it, then the shoulders have something to twist against