As a model trying to make this work. Shot after shot the hair doesn't separate enough to create a more voluminous spray. Any posing or movement tips?

https://quickshare.samsungcloud.com/zaQYRbSl74ZR

"...create a more voluminous spray. Any posing or movement tips?"
MLucie your example link isn't valid as of 01-18-2024 so I'm uncertain on what visual statement you're striving to create.
That said, I've collaborated with models on the aforementioned theme and what has worked well is having the model realize that they have to slowly "lift" their head out of the water and then gradually accelerate movement in a long sweeping arch.
Once they have this mastered it is up to the photographer to time the capture at the pivotal  moment of maximum dynamic motion latent with the hair. 

Not an easily done, but certainly doable with patience and persistence...
Hope this helps or is at least food for thought.
Wishing you much joy and success on your modeling endeavors MLucie