Wow, interesting thought. I never even considered a water-cooled AR-15. You would have to find a way to make the handguard water-tight, otherwise you'd have water leaking all over the place. And who knows that that water would do to the outside of your barrel, probably rust it all up after a while.
The old Maxims and Vickers water-cooled MG barrels never actually touched the water. The water jacket actually had a tube running through it for the barrel. Then there is weight concerns, steam venting, etc. The old water-cooled MGs put out quite a bit of steam once you heat up the barrels. A lot of them connected condensation hoses to prevent the steam from giveing away the MG's position. An AR-15 with a full length water-jacket would probably be to heavy to lug around for an extended period of time. The Vickers gun weighed something like 90 pounds, in that case, whats a few more gallons of water? Wouldn't seem practical to add a water jacket filled with water weighing more than the actual gun. Although a very interesting idea. May even be worth a look into for novelties sake. I don't think I have a water-cooled gun in my collection... hmmmm...