Out of air 

Scuba diving signals

As in the case of the signal above, this one is also quite sensitive andrepresents a serious problem in a dive. It is done by placing the palm facing down and simulating a cut to the chest or neck, to say that your tank has no air. If your buddy does this sign you have to give him your spare regulator (or pass him your primary regulator and you use your spare) and prepare for a joint water exit.