This is an extraction from Ken Rockwell's "How to Make Great Photographs" on how to produce different effects on water. Try it out!

TIME EXPOSURES FOR WATER

1/500 shutter speed freezes anything. This does not look as it does to our eyes. This is usually what you see in surfing magazines where every wonderful drop of spray is frozen.

1/30 makes moving water look about natural, if that's the effect you seek.

1/8 gives a nice blur.

1 second starts to get really smooth like this. Click here for sample photo

Several seconds or minutes starts to make all the whitewater flow into what looks like a fog. Click here for sample photo

In these examples the usual waves continuously came in and crashed on the rocks.