I've done waterfalls both ways, rapids, too. It depends on mood and what I'm trying to achieve/convey much like Mad Aussie said. I found the remnants of an old mill once and decided on a fast shutter speed because I think it conveys the 'brook as power source' idea more effectively than would a smoothed out shot. When I do want that silky water look I use a polarizer or a neutral density filter to keep my lens aperture in the sweet spot whenever possible. And yeah, a tripod is a must. Overcast or pre-sunrise light is also key, otherwise you get lots of blown out highlights.