The pressure goes roughly up by an atmosphere for every 10 meters you descend.
Go down 200 meters and that's 20 atmospheres and that's nearly 300 pounds per square inch.
The Titanic was 3,800 meters down, I make that 5600 pounds per square inch or over 2 tonnes.
You can attempt to make a hull that will withstand that pressure (like in a human habitable submarine) but you'll use a lot of steel, or you can equalise the pressure with gas on the inside.
Human subs have to be maintained at pretty much 1 atmosphere as oxygen becomes toxic at high pressures and surfacing would give you the bends but robots don't suffer like that so the best course is to try to to equalise the pressure where there would normally be any air spaces