Aqua Chief

The Aqua Chief glitch, which occurs in all games in the Halo Trilogy, allows players to walk around underwater, even though such an action would normally kill them.

Halo: Combat Evolved
On the Campaign level The Silent Cartographer, the player can go very far into the water around the island without dying. If the player walks out into the water, he will not be able to see anything, only his weapon and HUD. It is best to do this in a Warthog, as it is faster and the third person view allows the player to see which way he is going. It is easy to get lost.

Halo 2
In Halo 2's Campaign 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 over the peak. To get into the big lake, the player must corner ride down the hills and into the lake. Here, they can go into the deepest body of water in the entire game. At this point, they can see that the temple isn't connected to the bottom of the lake, and the player can go to the edge of the lake and beat the level.

Halo 3
It should be noted that in Halo 3, this is not a glitch.

In Sierra 117, at the river where Johnson's Pelican is shot down by Banshees, go to the edge of the dock's on the right. Go down the decline and into the river. The player can only move a certain distance before they reach an invisible wall. Note that enemies can still see the player, and if the Catch Skull has been activated, they will get to witness many underwater grenade explosions.