From Halopedia, the Halo wiki
The Aqua Chief glitch allows players to walk around underwater when such an action would normally kill them. In Halo: Combat Evolved, the campaign level The Silent Cartographer is designed so the player can walk and drive a Warthog into the water surrounding the island without dying.
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.
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.