Form is an element of art.
def. in art, the term form will be used to describe three-dimensional solids and contained spaces. In this context, form has both mass and volume, whereas, shape has only mass.
basically no 2D shapes.
I thought I could go with any 3D shap but I loved this because normally we see water as flat or not as visual as this. It looks almost like you could touch the water and hold it in your hand. Which means it has both shape and volume. It was a good choice of fast shutter speed (photography term) as it gives the viewer a different way to look at water. It provokes a more intense emotion, not calm and soothing like the ocean.
Please Note: since there is not required to do form I decided to take a try at it...just to see if I understood.