There has been studies with normal bongs and they mention some of the THC gets left behind in the water. With vaping it would seem you would loose a lot of cannabinoids because you are not heating the material up as much and a little water cooling effect could reduce huge amounts of your active ingredient. Unlike with combustion the cannabinoids were heated up right at the temp where they became airborne. Add a little cooling effect and a lot of it I think would just stay behind in the water.