it looks like they were succesful:
Beginning December 31, 2012, boats that do not meet the Environmental Protection Agency Rule for gasoline spark-ignition marine engines will be prohibited on Lakes Mead and Mohave. After that date, The National Park Service will require the exclusive use of four-stroke engines, direct-injection two-stroke engines, or equivalent technology. The public has almost a decade to convert to the new technology.
I found that here: http://www.nps.gov/archive/lame/station/news/030409.html