Four if by sea. (Updates).

Personal watercraft (PWCs), including Jet Skis, have long been the scourge of environmentalists because of the high levels of noise, air and water pollution they generate. As a result, PWCs have been banned from 66 of 87 national parks, Lake Tahoe in California and Washington's San Juan Islands (see "Raging Waters," In Brief, July/August 1996). The PWC industry has responded to the bad publicity by going greener. Yamaha introduced the world's first four-stroke engine watercraft last year, which was followed by models from Honda and Bombardier. These new PWCs are touted to be cleaner, quieter and more fuel-efficient than two-stroke models, meeting the Environmental Protection Agency's 2006 emission standards. President Tom Kiernan of the National Parks Conservation Association is still skeptical. "That doesn't mean they're non-polluting," he says. "There are less destructive ways for people to enjoy waterways." CONTACT: National Parks Conservation Association, [phone], www.npca.org.