It can be done, weld a bolt to the middle of the dent, heat the surrounding area up and pull the bolt, its rough but works, bike factories just fill the dent with braze and sand it down
Problem is its alloy so you need to find someone to Tig an alloy bolt to do it.
I had this prob on my easton tubed Kona U'hu. It was going to be too much hassle & expense ( frame was Karma'd to me)to send to a frame builder so i filled them with metal filler and had it sprayed.
It may not be as it left the factory but it looks great
I dont think it would work with alloy its very difficult to get the correct amount of heat as its too close to the Oh Shit moment when you melt the tube.
Best bet is to fill it and paint.
Never seen the water method used, I dont think it would work but I might be compleatly wrong.