Which gas is used by divers and why

What they breathe is compressed air. Yes normal air that every one breathes. This is compressed to 200bar and can last for approx. half an hour depending on the diver. What is exhaled is is also the same, namely carbon dioxide.

Sometimes divers mix gases to different ratios to give themselves longer dive times. This is called 'Ni-trox', which means (Nitrogen / Oxygen mixture). The gases exhaled are slightly different.