I've tinkered with the Usumacinta River tour for Google Earth that I posted a few weeks back. I fit Ron Canter's hand-drawn maps to the terrain a little better and changed the views on a few spots. Here's the revised version.
New York to the Usumacinta and Back (Revised 1/3/2006)
See this post for links to many more Usumacinta River maps, overlays, and tours:
The Daily Glyph - FAMSI Report - SCHERER: Tixan, SDL