Kai is correct regarding the ND filter...even a polarizer will help as it cuts down things by 2 stops or so.  Unless you are shooting in low light conditions (early morning, evening, overcast), the use of these filters will help you get the shutter speed down into the range to give you the water smoothening results you are looking for.

I tend to do things a little differently in these situations....since the shutter speed is the most critical factor in "smoothing" the water, I concentrate on it.  You can shoot in shutter priority, but I prefer to shoot in manual mode.  Set your shutter speed for, as Kai says a half second or so....then check your viewfinder meter to ensure that you are within the bounds of exposure...making sure that you are not over or under exposed.( Remember, the meter is your friend!)  If you are out of the exposure range, you will need to adjust your aperture accordingly to get you back.  If you can't get a small enough aperture at your desired shutter speed, you will need to further cut down light by the use of more ND type filtration.

What is the right shutter speed?....well, there really is no universal answer...it's really a juggling act, it depends on the speed of the water, the desired smoothening effect and, the limitations of exposure. The 6/10 second mentioned by Kai is generally a good place to start, but it may take some playing with different speeds to get what you want.

Last few points....set your ISO speed to the lowest possible and don't forget the tripod!

Have fun!

In youth, we learn....with age, we understand.