Halo Walkthrough: Under New Management
The starting video on this one is great - you get teleported in upside-down by mistake :)
You're on the Covenant ship looking for the control center. Run straight, grab the active amo on the left, then right, left and right. There's really no other way to go. You'll hit a hole in the ground. Just jump in. You'll be in green water land. Find the narrow corridor into the second big area. Let the guys fight each other. In the big water area head right through a narrow corridor. You'll find a med kit and sniper rifle. Another big fight is going on.
When it's done, go in and use the turret to kill the bad guys. When you go through and hit the pond with two waterfalls going in, go left up the ramp. Go around to the left grab the overshield on the right and then go around the left to go up to the gravity well.
Master Halo Walkthrough