There can be a wireless link between the diving cylinder and the surface team or the diver himself where they can know when the tank is about to run out. It can also be used to tell how the diver is breathing to tell if he is safe and is breathing normally.

GPS Tracking Device that tracks your location and sends its info to the people above water.

A foldout metal detector to find precious metals on the seabed and slightly underneath for a business venture or expedition.

There can be a salt measuring device, which alerts the diver when there is more salinity in the surrounding water.

There can be a small compressor to immediately start filling the cylinder upon resurfacing.