The Kahana is a ship that according to Naomi, is 80 nautical miles off the Island. She also says the helicopter is based on the ship. Naomi claimed that they are looking for Desmond, and is working for Penny. This was found to be false when Charlie contacted Penny, with her telling him it's not her boat. They are looking for Ben.

After the helicopter hit turbulence, it arrived on The Kahana. Desmond had suffered from "side effects", and was sent to the sick bay. Minkowski, the communications officer was also there, having the same problem as Desmond. They managed to reach the communications room, which was sabotaged by someone a couple of days ago. Minkowski dies as a result of the illness. Sayid manages to fix the equipment, and Desmond contacts his constant, Penny.
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Could it be that Kate is Jack's constant, Aaron is Kate's constant and Aaron is Kat'eKate's constant? I have a funny feeling that everyone off of the island is going to need a constant.

Personally (1stop4) I don't think people need a constant unless they have been affected by the 'side-effect'. I'm convinced Ben doesn't have a constant and I believe this to be because he's never been affected, maybe that is because his, and most people's, travel has been via submarine. Is it possible that submarine is the only way on/off the island without experiencing the side effect because it is airborne, that would explain why a submarine has been the preferred method of transport for sure.