I flew southwest about 6-7 times last season and i flew with my bottle everytime. Just make sure you stay there while they check your bag (if they take it an open it up). Wait on the side until you see your bag go through the xray machine, get stamped, and then taken away.
The nice things aboug southwest is that you can take 3 bags each up to 70lbs. With most other airlines its 2 bags each up to 50lbs.
If you think school is hard, try being stupid.