Sous Vide without vacuum. What liquid should be used?
If you look here, at the last paragraph of Yossarian's answer, he gives you a fairly easy way of removing the air from a plastic bag without a vacuum. 


  Put your food in the bag and then submerge the bag in water just to the zip. The pressure will expel all the air. Then zip the bag up as you pull the closed portion underwater. This gets results comparable with a home vacuum sealer, I think, with the added benefit that you can include liquids easily.


While I wouldn't consider myself an expert on this topic, I wouldn't fill the bag with anything you wouldn't want to marinate your meat in.