The problem isn't with the filter, its with the technique.
Your shot you linked was taken with a shutter speed of 1/15s. You aren't going to get milky water with that, you need slower exposures.
If you plan to do this during the day, you'll need a more powerful (darker) filter and a very small aperture. Its better to do it during and just before/after sunrise and sunset when the light is not as bright so you can expose for longer.
This shot for example was taken just before sunrise, and as you can see, the water is only just starting to get blurry and smooth. Shutter speed was 0.6s
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This photo, around the same time, shutter 3.2s, water is smoother and blurrier
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The longer your shutter speed, the smoother and foggier the water will be.