Smoke is an animated combination of gases, fumes and solids. The steam is almost almost all water vapor. The rest of it, at smallest amount in the case of ANSI issue marijuana circa 1975*, is as follows:

Gas Phase: carbon dioxide, carbon monoxide, acetaldehyde, HCN, acetone, ammonia, toluene, acetonitrile, isoprene, benzene, dimethylnitrosamine, and methylethylnitrosamine

Particulates: tetrahydrocannabinol, cannabidiol, phenol, o-cresol, m-p-cresol, 2,4 and 2,5-dimethylphenol, 1-methylnapthalene, 2-methylnapthalene, napthalene, benzoanthracene, benzopyrene

Out of this cloud, what you desire is the tetrahydrocannabinol, the extended and pedantic name for the notorious THC of legend. The relax of it is crap (with the possible exemption of the cannabidiol, which is a shut, but fewer vigorous family member of THC). Now, when you pass all this stuff through some water, you lose some of the water soluble constituents. Fortunately, neither THC nor any of its vigorous relations are water soluble. A bunch of the additional stuff, however, is, and transitory the lot through some water reduces the attendance of those constituents in the smoke. This is a high-quality thing as many of the water soluble constituents are carcinogenic and or toxic. Note that the lower the attentiveness of the constituents in the water, the better this works - so put original water in your bong one time in a while.