There is a pressure balancing diaphragm mechanism between the motor vessel and the sea water. Oil stays inand water stays out regardless of depth and relative pressures
if you design in such deformation zone everything will be hunky dory
Yes. It is done all the time on many bits of equipment which are intended for use subsea. It is called a Pressure Compensator.
Pressure vessels to keep subsea pressure out, are big heavy and very expensive.
If the "thing" which goes inside the pressure vessel, is able to withstand seabed pressure, then you don't need a pressure vessel.
All you need is a sealed container which sufficient strength to withstand normal mechanical loads, fill it with fluid (usually some type of oil) and have a pressure compensator which is usually a thick neoprene rubber bladder, which is filled with oil and connected to the container such that oil can move from the bladder to the container.
Then, when the whole assembly is put down on the seabed, the seawater squeezes on the bladder and the sea pressure is communicated through to the inside of the container.
Same pressure inside as outside of the container, so it does not need to be a pressure vessel.
Lighter, cheaper and everyone is happy.
JimB