As it turns out, it is, sort of.
In 1980 someone tried making a movie of Cussler's Raise the Titanic!, and it was pretty bad. Jason Robards as Admiral Sandecker? Richard Jordan as Dirk Pitt? The casting was awful, and the script didn't include Dirk Pitt's best friend and sidekick, Al Giordino, either.
Sahara, on the other hand, takes several elements of the novel and generates a synthesis of the story. It's been long enough since I read the novel that I don't remember the particulars of the story, but the entire premise is that the C.S.S. Texas, an ironclad warship, somehow ends up in the Sahara Desert. (In the book, Abe Lincoln's body is aboard it, but that was omitted from the movie.) And in the process of the evolution of the plot, the ironclad is found by Pitt et al, while foiling some criminal mastermind's plan for this-and-that.
I suppose I should find and re-read the book in order to compare the movie plot to that of the book, but I don't really feel the need to. Why?
Because the movie was excellent. It felt like one of Clive Cussler's over-the-top adventures. Cussler has a wonderful ability to write outlandish scenes that nonetheless have the disbelief-suspending properties of a heavy-lift dirigible, and as silly as some of the plot synopses seem on the surface, they always make sense when you read the whole book--they make sense and they are believable. ("Believable", that is, in the context of an adventure yarn. Cussler is a step or three back from the hard-real Tom Clancy paradigm--but it's a fun step or three.)
The best example comes in the climactic battle scene, where Dirk, Al, and Eva shoot down the bad guy's helicopter...with a 150-year-old cannon. There's no way in hell that would work. The cannon would explode, the gunpowder would be useless after 150 years in the desert, the cannonball would be a dud (probably, anyway)--and yet the movie makes us believe in it happening, much the same way Cussler can make us believe some of the crazy-ass stuff in his novels: they make it entertaining.
And that's what made it work: the movie was fun. And not only was it fun; it was fun the same way Cussler's books are fun.
Is Matthew McConaughey the best actor to play Dirk Pitt? Probably not, but he did a good job at making me believe he was. Steve Zahn did an excellent job as Al Giordino even though Al is supposed to be a swarthy italian, not an affable blonde guy. (William H. Macy as Admiral Sandecker? That was a non-obvious stroke of casting genius. It worked perfectly.) But the cast worked, even in the places it was suboptimal, because the casting director got the right kind of people for the roles.
So Sahara isn't exactly what Clive Cussler wrote--but it is, essentially, a Dirk Pitt story, and a pretty good one at that. I think I'm going to buy the DVD.