If you have them you can stack a circular polarizer and a linear polarizer; if you don't have them you're better off to buy a Vari-ND or equivalent.
I agree. If you intend to take a reasonable number of water shots over time, the Singh-Ray variable ND filter is great. More than eight stops darkening is possible, and you can dial them in to bracket your shots. No matter how much experience you get shooting moving water it is always nice to twist a dial, shoot, twist a dial, shoot, and almost guarantee that one of the exposures will be just what you want. I don't use mine that often but when I do, it makes it much simpler to find the right shutter speed for the effect I want, which would vary with the speed of the water. Whether there are parts of the water moving at different rates, the angle of the camera to the water, etc, are all variables that affect what might falsely seem to be a simple decision concerning shutterspeed.