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.
Not sure what your shutter speed was with the first one, but with a ND8, and low lighting, I suspect it would be slow. I'm surprised you didn't get more motion blur off the people in the picture. Only a couple of heads that I saw. Nice first attempt.