It may enable you both to breathe and to view a switch. After triggering a switch, you must retrieve the Golden Key from a closed chamber between the two switches. Once you have the key, work your way to the lower-right map corner. Climb out of the water and go to the right to a red doorway. Go through the door and continue right, to an explodable stone wall.
Death cavorts on the ledge above.
You must deal with it, but you can do so after pulling the switch. Climb to the ledge, run past Death, and scale the ladder. Flip the switch and then climb back down the ladder to destroy your archenemy. Climb back down to the stone wall and blow it up. Flip off of the ledge, kill Death, and then follow the path down two ladders and left.