Cycling in Sydney Australia
I have it direct from Qantas's dangerous goods department via my check-in assistant that yes, yes you can. You can carry up to four either cylinders up to either 25g or 28g each (I can't remember which, but it certainly covers the more common sizes) in your checked or carry on luggage. She did suggest placing them in my carry on rather than checked luggage to avoid getting called back from airside by security if my checked luggage was screened (he says with a shudder, still remembering the time he had four uniformed officers who didn't speak much English find him in the lounge in Berlin Airport and request he follow them to a small windowless room where my suitcase was sitting on a table - note to self: don't pack your computer mouse in your checked luggage).
When the CO2 canisters went through the x-ray machine in my carry on luggage I was asked to have my bag searched, but that was all - once I'd shown them I could carry on (NPI) as normal and didn't have to surrender them.
Of course, your airline may differ, but if you use CO2 (which I do, but didn't need to on this trip) it's certainly easier to be able to carry a few with you than try and find somewhere to buy them when travelling. That being said, I can highly recommend the Lezyne HP (high pressure) pump as perfect for travelling - small enough to fit in your suitcase and jersey pocket (although not exactly tiny) but powerful enough to get to 100+PSI without too much effort, and with a gauge so you know when you've got there.
Perhaps you could also put them in the separate container along with keys and wallet, that way you don't even need your bag searched.
There is a small CO2 cannister in the self-inflating life vests under every airline seat.
and water comes out of the tap but they wont let you take a full water bottle through security....
If you both had the time, materials, and inclination to prove to them it was water and nothing but water, I'm sure they would. I flew back from Queenstown on Thursday, and the lounge was a shared one for several airlines both domestic and international, so is before security. The last thing I did before leaving the lounge was fill up my water bottle; the next thing I did was tip it down the toilet sink before being allowed through security...
So what you're (not) saying is, don't bother taking any with you, just reach around under the seats in your row? ;-)
Which airline? You frequent flier miles may vary...