If you choose to check the MSDS(Material Safety Data Sheets) for listerine, it will tell you all you need to know. it is safe as long as you don't steadily inhale it for too long. If you do it can cause headaches, dizziness, and irritated lungs. I would suggest mixing it at least 50/50 water so it doesn't irritate so much. it actually sounds like a good idea. Listerine is pretty low in alcohol and the smoke is only in it for a very short amount of time so you wouldn't really have to worry about losing THC. Also it might help reduce the amount of impurities you might breath in as well as keeping the bong clean and reducing the smell. it's also not strong enough to "explode" especially if it's watered down.