What the halo 2 water wall is just a wall I and many many many other people found that if you fire at it, it will be like firing at water (its really cool so check it out!!!)
Step1 Get sputnik skull. Its easy just go to level "Quarantine Zone" on Legendary you just start the level and turn around go into the cave and then when you are in the open area go over to the left side of the 'bridge' thingey and you will see ledge and then you go walk on the ledge until you get to the end of it you will see a flying skull hold X to pick it up and there you go you have sputnik save and quit after you pick it up.
Step2 Get to roof top now that you have sputnik you can get to the water wall now go to the level Outskirts play through the level untill you get to the beach like area you play until you get to the jackel snipe stand things (it hovers in the air you cant miss it) you jump on top of the snipe thing and you through a plasma grenade in the senter and grenade jump over to the roof and you should make it if you did it right.
Step3 Geting to the wall. now that you are on the roof top you will see a hole lot of nothing (your xbox is not broken you glich noob) you can look around if you want (it gets boring REALLY fast because thar is nothing to look at) ok now back on track keep moving foward and than take a right the first chance you get jump up the orange/rust colored thing and take another right then take a left and you will see the next area DONT jump down turn to your right walk forward till you hit a wall than if you look to your left and you will see a wall with a very weird design and if you can stand to look at it for to five minutes you can fire at it and look its like firing at water!!!
Step4 Find errors (if any) if i messed up please tell me as soon as you can i would love to fix it and make this better for everyone.