Technologies
There are three basic technologies used in the design of submersibles. Single atmosphere submersibles (one atmosphere subs) have a pressurized hull and the occupants are at standard atmospheric pressure. This requires the hull to be capable of withstanding the high pressure from the water outside that is many times greater than the internal pressure.
Another technology called ambient pressure maintains the same pressure both inside and outside the vessel. This reduces the pressure that the hull has to withstand.
A third technology is the "Wet Sub", which refers to a vehicle that may or may not be enclosed, but in either case water floods the interior so SCUBA equipment is used to facilitate breathing. In both single atmosphere and ambient pressure subs, there is no need to use SCUBA equipment and occupants can breath normally without wearing any equipment.
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