A buddy of mine had things all set up for three days on a live aboard to the Dry Tortugas. He arrived, and was told that they had canceled the trip (or maybe the canceled on him a couple days before he left) because a couple of divers backed out. They would not go out with just a few divers, they wanted a full boat, or divers willing to pay what it takes to make the trip worthwhile.
If you have the bucks, I'm sure you can find a live aboard for 2~4 divers. I personally don't know of any op that would go out with so few at standard rates.