Anyone with knowledge of general level chemistry can easily figure out that THC has extremely low solubility in water. Solubility has a lot to do with inter-molecular force (van der Wall's forces of attraction), and can be generally determined by the rule "like dissolves like." Water is a polar molecule held together largely by a molecular force known as hydrogen bonding, the most powerful inter-molecular force second to ion-dipole attraction. Hydrogen bonding describes the repeating bonding chain of Flourine, Nitrogen, or Oxygen atoms with Hydrogen atoms. The H2O molecule easily rotates to accomplish this when bonding, forming very long chains of O-H-O-H etc bonds. Things are soluble in water because they likewise form hydrogen bonds, and those bonds are strong enough to break H2O-H2O bonds and suspend the solute molecule in H2O solvent solution. However, the chemical composition of THC negates any kind of solubility it would have in water.
The composition of THC is C21H30O2, and its structure is formed by long hydrocarbon chains. These chains consist of hydrogen atoms bonded to carbon atoms, and due to hydrogen's 1s1 valence shell arrangement, it cannot accept any new bonds, nor can it form a hydrogen bond with a carbon atom in the chain. There are 2 oxygen molecules, but neither are in proper position for a hydrogen bond. This leads to the chemical conclusion of why THC is not soluble in water. While it is polar and has significant London dispersion (inter-molecular forces), no hydrogen bonding exists and thus the "like dissolves like" rule cannot be fulfilled.
And, in case you are curious and still reading, there is a chemical reasons for why bongs fuck you up so much harder than pipes and joints. The first is the polar force of H2O. When marijuana is burned, it decomposes into about 200 chemicals, and when passed through water, they decompose again into about 200 more chemicals, for a total of 400. This disassociation allows the molecules to bond much more easily with the inside of the lungs (due to their smaller size and willingness to bond) and thus more THC "sticks." Another reason has to due with gas laws and the effect of temperature on the smoke.
According to Charles' Law, volume/temperature= a constant, and when temperature decreases, so does volume. Therefore, when the smoke passes through water and over hopefully over ice, it condenses, thus fitting more chemicals into a smaller space (I can explain the reason for that too, but it's a different story). This means that more chemicals are inhaled.
And finally, the last gas law relating to a bong is the effect of temperature on gas solubility. Since molecular motion is slowed by the decrease in temperature, molecules have a much better chance at sticking to the walls of the lungs. I have asked myself the molecular reasons behind bongs more than once, and I hope this was of interest and answered once and for all the question of THC and its solubility in water.