Decorative vine that likes water/ponds?

I have a waterfall that pours into a pond that is filled with rock (6" to 10" granite and black basalt); I'm looking for a vine that will creep across the rocks and enjoy the ample supply of water. The water level is about 6" below the rock layer so the vine could send runners into the water. Does anyone know of a good plant to do this? It's in the Pacific Northwest, something as cold hardy as ivy would be appropriate for our climate. Thanks!