I think there are very few (if any) fish that can survive at that depth. Most fish have an air bladder that inflates and deflates to keep them neutral. You see some crustaceans and mollusks in deep water, but at those extreme depths the life forms get really, really weird. Alien shit.
I think what you're thinking about is a suit that allows for breathing an oxygen-rich liquid that your lungs can exchange oxygen and CO2 with directly. Since it's not compressible, your lungs can be at ambient pressure. There have been stories about that, but afaik, they are all just stories. I think training yourself to intentionally drown would be hard.
We solve the problems with submarines. It's just expensive to do safely, and just like space travel, there is no margin for error.