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 heads-up display. 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.

It is also possible to grenade jump with the Warthog and get out of even those barriers.

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, beating the level.

Halo 3

 * 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 may get to witness underwater grenade explosions.
 * You can also do this on longshore by jumping off a Man Cannon at 50% gravity facing the water, you will slide down the water and be able to hover on the water for about ten seconds.