As you know the main limit of how far a submarine can dive is determined by how much pressure it can withstand per square inch, exceed this and the sub will crumble under the weight of the water. The main method used to counter the pressure is the structural integrity that can equally distribute the pressure so there's not a single point of weakness that can crumble, then comes how much weight the structure can handle also.
The idea then is to increase the integrity by supplying an external force that can counter the force coming from the outside the same way the sun has in the core the fusion reaction that pushes matter out countered by the force of gravity trying to collapse the matter, this interplay leads to the core heating up to a high degree that fusion reacts and produces the energy that is used to push matter and energy outwards keeping the core stable.
In a similar manner designing a submarine that uses electromagnetic forces which is one of the strongest forces. The concept is having the ship's inner core having a strong electromagnet which has its dipoles (North and South) matching electromagnets also lined on the inner side of the hull of the sub. When the core's electromagnet is turned on and the hull's magnet the two would repel and force the hull to be pushed outwards. Given the core is set like the sun's core, a ball that can handle the incoming force it would be stable, also the outward electromagnetic field would also need to supply an equal amount of repulsion such that it doesn't apply too much on one part making the hull erupt from the inside. Given this as the case the hull will be exerting an external force on the water and therefore the pressure the water exerted by the water would be that minus that exterted back by the hull. The more pressure exerted by the hull outwards, the less pressure the water is able to exert back and therefore the less pressure experienced by the hull so the deeper the sub can go.
Theoretically, based on the assumption all this holds true, the deepest the sub can go will depend only on how much electromagnetic negative pressure can be exerted safely.
The added bonus is with metal like transparent aluminium, three times stronger than steel, adding a viewing area with light coming from inside the sub is possible with dives done with automated control to test safety before finally having a manned mission to the deepest parts of the ocean.