Hot water or steaming water or whatever SUCKS. I've tried it on a few occasions, and have NEVER had a good result. You're also likely to put some overly-hot water in the bong and shock it, making it shatter. Cold water is the only way to go, thats why it's the most common thing used.
I know how bad this sounds, but trust me on this one, the best liquid is by far urine. The active alkaloid dehibiting enzymes trigger chemical responses in the delta-9 THC converting it to far stronger cannibinoids that hit harder and last longer. The best kind of urine is from your common house cat, haven't you ever heard of the bud strain Cat Piss? Here's how it got its name. Also, it's been shown that adding a tablespoon of semen in there too can increase the efficiency by roughly 32%.
When my friend has a cold he takes cough drops and dissolves them in water on his stove and uses it in his bong.