Cool! I got started on Clive Cussler mooching my dad's books as a kid. As an adult...I still mooch my dad's books
Dirk Pitt has a "scruffy" aspect to him, and an interesting blend of dark cynicism and love of life. I haven't seen Clooney convey the love of life aspect in his roles; Dylan McDermott isn't scruffy enough. Hugh Jackman -- he might work.
Al Giordino -- that's tough. Need a short, heavy, strong, Italian guy, who can go from "let's party" to "let's rumble" in an instant.
Sandecker -- I'd go with James Earl Jones (even if he isn't black in the books; or is he? I forget). Or the bald guy on the TV show JAG. Actually, the main actor on JAG could be a decent Dirk Pitt.
Regardless, I hope they do a good job with the movie. This could be a killer franchise!