I have actually done some reading about the 747 tanker. In theory, it's a good idea. BUT, tankers like that are mostly used where firetrucks have trouble getting, like steep canyons. The problem with the 747 is that the airplane is so big, there's no way it can get down low enough to make a difference. The retardant gets too dispersed to actually do alot of good.
There are several variations on the story. WHen in doubt, visit snopes.com for the straight skinny. Here is the article from snopes on the diver story: