All Navy SEALs are required to undergo specific and required training in various types of scuba equipment.   They will use this scuba equipment based on the specific Navy SEALs missions.  The three most commonly used types of scuba equipment among Navy SEALs are: the open circuit system, the closed circuit oxygen system, and the closed circuit mixed gas system.

The open circuit system is the most used variety of scuba equipment used by Navy SEALs.  This is a standard breathing apparatus where a SEAL will take in air from a tank that they are wearing.  This type of scuba equipment is generally seen being used by amateur scuba divers.

A closed circuit oxygen system is very different than an open circuit system.  This type of system causes the user to breath in 100% oxygen.  When it is exhaled inside the breathing apparatus the oxygen is filtered and turned into air that is breathable.  This type of scuba equipment will generally be used by the Navy SEALs when they are operating in shallow water or in very cold water.   But when diving using this type of scuba equipment a Navy SEAL will also have to wear a dry suit.

The closed circuit mixed gas system works just like the closed circuit oxygen system, only the oxygen in this type of scuba equipment is mixed with air to create a specific partial pressure of oxygen level.   This type of scuba equipment is used when a Navy SEAL will be in great depths of water.   Using this type of oxygen tank also allows the Navy SEAL to stay underwater for longer periods of time.