From what I'm reading in the official forums, Forza's puddles do not interact with the wheels, you glide over them every time. Even cones float on top of the "puddle". It's more akin to black ice, so really, what is the point of a "3D puddle" besides an obnoxious loss in steering like you always hydroplane...
While there is no three dimensional representation of water displacement, the physics of the situation are actually modelled. If you go slowly through puddles you do not hydroplane, where as if you go fast over them you do.