I have wondered since my first viewing about that darn cable or pipe or whatever it was going into the water. I also have wondered and theorized (among many) about an underwater hatch.

But i don't think its likely it goes to the hatch Jack is in. Maybe another one? When we see the large view of the island when the plan crashes in this episode, the island is massive. That isn't to say where ever they have taken Jack isn't closer to the Losties, but it seem like the location of the underwater hatch isn't close enough to be the same hatch that cord goes to.

Now, like i said, the hatch Jack is at could indeed be VERY close to the Losties (which would prove most interesting)....but who knows right!?