The Aqua Chief is a glitch that can be done in all the games of the Halo trilogy.
On the level The Silent Cartographer you can go very far into the water around the island without dying. If you walk out into the water you will not be able to see anything, only your weapon and HUD. It is best to do this in a warthog since it is faster and the third person view allows you to see which way you're going.
For Halo 2, go to the level Delta Halo, go up the ramp in the mountains to the right and jump up onto the slanting hill to go up. From there, wind around the mini lake and go up the second biggest mountain and go over the peak. To get into the big lake, you must corner ride down the hills and into the lake. There are 3 things you can do then, you can go into the deepest part of the entire game or maybe the entire series! You could see that the temple isn't connected to the bottom of the lake, or you could go to the edge of the lake and beat the level, and you don't die anywhere under the lake.
Now, to Halo 3. This glitch can be done on Sierra 117. You go to the river where Johnson's Pelican is shot down, go to the edge of the dock's on the right, go down the decline, and go into the river, you can only move a certain distance before you reach an invisible wall. Use your imagination on what to do after that. You should remember though, that enemies can still see you, and if you have the catch skull, prepare to see a lot of "Aqua 'Nades."