I would think that hot water would just make it a terrible experience. Like everyone else said I think it would only make the water smoke hotter then you would have a hard time holding your bong cause it would heat up too. Also if you are smoking the vapors from the water I am sure that wouldnt matter too much cause they would be hot too and just make it harder to keep smoking. I would definately keep with the cold water cause the whole purpose of the bong pretty much is to clean up the smoke and cool it off so it isnt as hot.