Sorry for the typo and lack of clarity! That is 11m/(s^2) for Neptune's gravity approximation. This value was just to point out that the pressure difference due to gravity could not be much different then 9.80665 m/(s^2) or earths gravity. Things tend to be denser at higher gravity levels. Example is Jupiter with a much different gravity level changing variables a bit too much... For Neptune, I think mainly temperature does matter because it determines at what pressure the atmosphere turns into an liquid. For example, why would there be any gas vapor in the atmosphere at -200 Celsius if the pressure was at 1 bar? As you descend from the vacuum of space, the pressure increases and increases until you reach 1 bar. I am saying on the way down into the increasing pressure of the atmosphere, gasses would liquify at far smaller atmospheric pressure then Earth due to the low surface temperature. Before you descend into 1 bar atmospheric pressure, you would land in an ocean. As long as we are at 1 bar, we should remain less dense the the liquid form of any gas that is cooled into liquid form assuming the gasses we breath and our cells are still relatively warm. Perhaps, if the gasses in the lung are also at -200 Celsius thought, we would sink.