Scuba diving is an underwater activity where a diver uses a self contained underwater breathing apparatus to breathe underwater. This differs from snorkeling and free diving where the breath is hold and time underwater is limited by breath hold capability/capacity.
Scuba diving includes Semi-closed Circuit, Closed Circuit and Open Circuit (OC) apparatus. Semi-Closed Rebreathers (SCR) and Closed Circuit Rebreathers (CCR) extend time over the normal Open Circuit diving by using scrubbers to remove Carbon-Dioxide from the exhaled air, allowing the diver to re-breath the same air and also by keeping an optimal Oxygen partial pressure during the whole dive allowing for extended dive time as less nitrogen is absorbed.
Standard Recreational Open Circuit equipment include: Compressed Gas Cylinder, Regulator (1st stage regulator, primary 2nd stage regulator, alternate 2nd stage regulator, submersible pressure gauge, Depth Gauge, 1 or 2 Low Pressure Inflator Hose(s)), Buoyancy Control Device (BCD)) and the soft gear which can include mask, snorkel, fins, weight belt, wet, semi-dry or dry suit.
There are various organisations doing training for Recreational and technical scuba diving using OC and/or SCR or CCR