It's most likely not going to kill you if you give yourself a bongwater enema (that said, stale bongwater is likely to be full of all sorts of lovely bacteria that you probably don't want to introduce to your mucous membranes), but as best I understand it there's little or no dissolved THC in it either. If I'm remembering this properly, THC is a nonpolar molecule and thus requires a nonpolar solvent to dissolve it. Water is a highly polar molecule (the oxygen is way more electronegative than the hydrogens are, so you get a fractional negative charge on the oxygen and fractional positive charges on the hydrogens), and so the only way to trap it in your bong would be to use alcohol (which is extremely flammable in high concentrations and the vapor is really hard on your lungs) or PEG-400 (which is most likely way too viscous to successfully pull weed smoke through). Unless I'm really far wrong, the only highs you're likely to experience from a bongwater enema are traceable to either the placebo effect or neurological damage due to bacterial infection.