Do you mean a CPL, Circular PoLarizing filter?
The front element of a CPL filter needs to be turned as you look in the viewfinder. You are looking to adjust how much effect you want.
With a CPL the light angles are important.
The ND8 photo looks like you had mixed lighting and your white balance was off because the photo is very blue, but the filter allowed you enough shutter speed to blur the water jets nicely.
ND filters are usually rated by stops with .3 being 1 stop, .6 being 2 stops, .9 being 3 stops, etc.