In the liquid analogy, the equivalent to voltage is pressure, but you can't compress a liquid, you can compess a gas and you can force more charge into a conductor, just like with a gas.
Again, no analogy is perfect and they all fall apart depending onw how much you want to prod at them. Take for instance this scenario- I want to transmit one coloumb of charge across a wire- no problem.The size and length of the wire really don't matter. What if I want to flow 1 tablespoon of water across a 100 foot garden hose? what goes it may not even come out. The liquid and gas molecules themselves have volume and must move to go anywhere, unlike with electricity where the electrons going in a wire are not the ones going out and you're not moving any materials.
Not off hand, but this guy really gets into the topic
if you get really bored there's more complaining
I'd disagree. You can always cram more gas into a vessel, but only so much liquid, no matter what the pressure. I can't stress it enough, but all these analogies do fall apart, I just feel the gas is more accurate than the liquid one for most folks.