the same thing everyone breaths, oxygen
water proof ones
In recreational diving, most divers use compressed air (as in the normal air we breather at the surface). Some divers are also trained to use special gas mixes called Nitrox which have a higher oxygen content and lower Nitrogen. Professional (or commerical) divers tend to use more advance mixes such as Heliox or Trimix which combine Oxygen, Nitrogen and Helium.
ALL gasses can cause the bends (even oxygen in extreme levels). But for most divers it is nitrogen and for deep mixed gas technical divers it can also be helium that they added to their gas mix. However, it is not the blood divers are worried about (even though that is the transport mechanism), it is the gas that expands in the cells and tissues.
What kind of companies use natural gas in California?
Some diver use nitrogen to stay underwater for longer but usually compressed air is usedfor those who don't use nitrogen