Water insoluble doesn't mean complete insolubility, only really low solubility.

From Wikipedia:
Most smokers believe that bongs are healthier than other smoking methods; however, a 2000 NORML-MAPS study found that "water pipes filter out more psychoactive THC than they do other tars, thereby requiring users to smoke more to reach their desired effect" [5]. Smoke from cannabis supplied by the NIDA was drawn through a number of smoking devices and analyzed. A smoking machine, adjusted to mimic the puff length of cannabis smokers, drew smoke through a standard bong, a small portable bong with a folding stem, a bong with a motorized paddle that thoroughly mix the smoke with the water, and two different types of vaporizers. The study found that the unfiltered joint outperformed all devices except the vaporizers, with a ratio of about 1 part cannabinoids to 13 parts tar.