At the end of the level, when the Pelican arrives, itdestroys the two Covenant ships; one being above the water just outside of where Johnson is held. The other one is above the water in the place just before the Pelican lands. Climb in. Once the Covenant ship is destroyed on the side where it picks you up, the debris that remains will float on the water. Jump onto it and let it sink. From there all you can do is just jump from one piece of debris to the other. Note: If you touch the ground you will die.
At the part where the dropship pick up goes wrong (the Banshees attack the Pelicans), you are able to breathe underwater and get the beam rifles (under the Legendary or Heroic difficulty settings only) on the other side thanks to the Jackals. Note: The river also makes it so the enemy cannot hit you.