The few times I've experimented with slower shutter speeds and water, (not often as I don't have a tripod yet so it is an awkward process where I have to put my jacket on a rock somewhere and then try to nestle the camera in it) I didn't go nearly so slow. Maybe 1/8 second at the most.

I agree that the above looks better at 1/500 but I'm keen now to try again somewhere with a really slow shot (yet another indication that I need my tripod sooner rather than later).