Image:5D Mk, 24-105 f4 L IS, Variable ND Filter, 20s exposure at f/11.
Image:40D, 24-105 f4 L IS, Variable ND Filter, 1s exposure at f/11.
Image: 5D, 24-105 f4 L IS, Variable ND Filter, 25s exposure at f/9.
Image: 5D, 24-105 f4 L IS, Variable ND Filter, 30s exposure at f/11.
To smooth out water or slow it down, you need to reduce the amount of light reaching the camera sensor, a quick and easy way to do this is by adding a neutral density filter, the most versatile that I have found is the Variable ND filter made by Singh Ray it allows for 8 stops reduction in light far more than using a CPL or other type of ND filter, it takes a little bit of experimentation, but it can be a lot of fun.
Results can mean images far beyond your expectations. The use of a sturdy tripod is a must.
Learn how to use the bulb mode on your camera for even longer exposures.
Images In Light