In the light I had, this meant that I could leave the shutter open for about 6 seconds. The settings I used here were iso 100, 6 secs at f/22. This averaged out the water movement giving it a velvet appearance - I like it in some situations because instead of capturing a snap of the water surface, it shows water ripple trails, showing water movement.
If you want to experiment, you dont need to go to that expense though :) you can hold a variety of different mediums in front of a lens to get a similar result - some examples would be colored glass or (if rigid plastic) clear medium with thin washes over them.
The technique can be overused - sometimes it is just as good to catch the water flowing without time lapse.
(Thanks to +Renee Leach - who asked me how I did this.)