I agree with it being possible. The woma python has been crossed with the ball python several times and its amazing not only that they are from different genera but the woma has no heat pits and looks more colubrid than python. Also the black rat snake has been crossed with the twainese beauty snake, these two snakes are seperated by millions of years of evolution compared to the only several thousand years water snakes and garters have been seperated. The odds for these two crosses were also incredibly unlikely yet they have been proven. This I think makes the likely hood of a water x garter snake much more likely.