You've got to watch this video. I love these dudes. Originally from the Map Room.
The project uses an innovative underwater video system that takes Global Positioning System (GPS) digital data and stores it continuously on the audio track of the DVD as the kayak floats down river.
Simultaneously, the river’s surface features are recorded using a similar geo-referenced video camera. Images from both video cameras are downloaded into a Geographic Information System (GIS) to produce digital maps that depict the stream in minute detail, above and below the surface.