We do use the cloud density, a cloud is both water and air. Just water or the air alone wouldn't be a cloud.
You can read
this thread where our cloud density changed, or
this thread that is similar to your own thread.
While not both the air and water would move at equal speed due to their difference in density/mass, there is currently no possible way to tell how fast the water moves compared to the air and vice versa. Ignoring the air and using only water or vice versa would be completely absurd.
If that doesn't satisfy you... Well that's all I can do. Maybe someone else can address your concerns.