I think that it’s implicit that the volume of Godzilla would increase; we need to assume that the bounding layer has a degree of elasticity and that that the matter displaced by the flotation cavity will expand into that, reducing the net density.

Mighty Godzilla, with power untold

Rises through the waves; his powers unfold

Hidden muscles in clever design

Create a new chamber as they realign

Inflating his body, a titanic display

Defying the depths, he floats up and away

No long bound by the oceans’s might

Godzilla soars, a triumphant sight!

You are right that you need to dehumidify a large volume of air (like, more than 150 cubic metres) to get enough water for a person, in a rationing scenario. That isn’t necessarily a huge problem; fans can move large volumes of air quite efficiently and dehumidifiers work more efficiently at higher temperatures, but your main point that it isn’t scalable on a population level is definitely true - it’s something you can do for off grid or survival situations, but not easily for domestic water supply.

That’s 17g or 17cc per cubic metre, not 17 litres!

In dry countries the relative humidity can be low - sub 40% is common. That said, the absolute water carrying capacity increases rapidly with temperature, so at 40C, the saturation mass is more like 50g/m3 and at 40%RH it still carries more water than 100%RH air at 20C.