I looked last summer and the only other operator was Shannen's Scuba Safari. They are a private charter near the cruise port and were charging $2200 for 6 divers. I haven't checked in a year but I doubt anything has changed. Blue Adventures Excursions at the Atlantis Resort lists diving on their website, but they use Stuart Cove's.
@BabyLitigator I would take the official shore excursion. It might be a little more expensive than booking directly with Stuart Cove's, but the transportation will be included and if anything happens you won't get left behind.
Unfortunately since Stuart Cove's moved to the far side of the island, it's gotten harder to dive with them. It looks like it's a 30 minute drive (factoring in traffic). On their website it used to say if your cruise ship didn't arrive by 7a you couldn't dive in the morning with them. 9a is definitely too late to make their morning dive. Depending on when you leave, you might be able to make an afternoon dive. For those that haven't cruised before, the all aboard time is 30 minutes earlier than the departure time listed on your itinerary.
You should expect that you won't actually get off the ship and ready to get into a taxi for 45-60 minutes after the ship arrival time. The ship has to dock, clear customs, then everyone get off the ship. Sometimes it goes smoothly, but sometimes not. You might get off sooner but I wouldn't plan on it.