whether you claim electrostatics gravity or density as a cause of the downward vector, you are claiming that the water bottle is stationary and when let go its the bottle and its contents moving towards the ground, by claiming such you are claiming that the bottle and its contents are being accelerated towards the ground, in every case of acceleration of a water bottle, we see displacement of the liquid and gas, however, when we let the bottle go, no displacement occurs therefore there is no downwards acceleration, unless you want to invoke fantasy tractor beam technology that can accelerate the bottle and its contents without displacement occuring, leave the fantasy physics for the movies, thats not how reality works