You are looking at the tank attached to the test vessel top. The tank is suspended from this and is lowered into the water. So, the tank is completely filled with water and the chamber outside the tank is filled as well. The top is attached to the vessel lip with the pneumatic clamps. There is an O-ring between the them to insure that water cannot leak out between the two.
Any air is purged from the test vessel.
There is a hose to the back left. It supplies water to the interior of the cylinder. It is also the source of the high pressure that will be applied to the tank. The scuba tank is enclosed in this vessel. Both are filled with water and pressure is about to be applied to the scuba cylinder.