The fastest way out seems to be to climb the well, so you slowly creep forwards.

The sloshing water sounds unbearably loud in this enclosed space; you keep watch on the worm as you approach the yawning tunnel. The worm faces you directly and you freeze, but breathe a bit easier as it continues scanning the room. It seems that the worm can't see anything in this darkness. Hopefully it can't hear you either...

Unfortunately, it doesn't need to. As you are about to reach the other side of the room, the ripples you make in the water reach it. It whips back to face you, then dives into the inky water and disappears. No doubt it's heading right for you! There's no point using stealth now - you have to fight this thing off, or make a run for it.