*Technically* THC is slightly soluble in water, but not enough so that we can make any use out of it. That's why you can use water in making hash. The THC molecules are trapped between the water molecules and when the solution is passed through a filter, THC resin is left for you to make into hash. If THC were completely soluble in water you would be left with nothing in the filter and a THC solution down the drain. Same deal with water curing. If THC were water soluble you would end up with a shitty plant that wouldn't get you high because all of the THC would have been leached by your curing solution.
So for general purposes we say that THC is not water soluble. THC is, however, soluble in alcohol which is why you can make a tincture that can get you pretty fucked up.