It really depends on the light condition, if you want to shoot waterfall, maybe a circular polarizer should be enough unless the condition is really very bright. It will be good to have a ND filter to standby, not really a must.
As for ND Filter, here is a list, hope that it helps.
ND2 - Reduce Light Entering the Lens by 1 f-stop
ND4 - Reduce Light Entering the Lens by 2 f-stops
ND8 - Reduce Light Entering the Lens by 3 f-stops
CPL is to cut off the glare and unwanted reflections (although it does help to reduce the amount of light entering, it does not consitute a ND)
ND's as the name implies is a Neutral Density filter, meant to reduce the amount of light without interfering with the colour cast. A graduated ND will however help to show up some parts of the scene to be darker and make certain scenes more moody.