A few of my water based renders have taken a long time to render. I think my record is 144 hours when I was using Vue 5 way back when. It was of a surf environment, with a multitude of textures,  Eight lights (Some volumetric) Everything anti-aliased. Which, in retrospect, was not the most efficient way to go. The final result, although adequate, wasn't enough for me to show it in my gallery. (Picture attached) Keep thinking one day I'll go back to it and do it up right.

The water in this image took over forty hours:
http://www.renderosity.com/mod/gallery/index.php?image_id=1531205&member

...Insert some witty or thought provoking comment here...