Submarines in barotrauma are rated for a depth of 1600.
The optimal depth is around 1000-0 usually. Past 2000 meters the submarine will take substantial damage to itself.

There are monsters and scary things in the Abyss, but what lies beneath it?

To find out, we can add Submersibles which are special types of shuttles, however usually much much smaller and made to be unguided or have 1-2 crew members.
Many devices do not operate at the depth that they go on(?), so they are essentially dark aside from limited sonar.

shallow idea, but I thought it's worth noting.

It would actually be cool to explore the very bottom of the abyss.

Maybe alien ruins should be down there. They are like everywhere in the current game so maybe they should be harder to reach. Maybe have some ruins that aren't completely sunken. Oxygenite Shards lying around could also cause some of these non sunken areas to have breathable air too.

There could also be new threat types. Maybe disembodied entities of some kind. They would be unable to cause physical damage but maybe they could press random buttons, produce sounds like human voices or maybe cause hallucinations like making you see one of your dead crew members or random ones or a creature, only to disappear when it's close enough.

Or we could just have some more good ol human eating sea life.

Actually you can make a submarine capable of going deeper by using exclusively heavy walls in its outer hull. Walls with higher health can withstand higher pressures in-game, and heavy walls have twice the health. Can't remember the exact details of how it's calculated, but i think you should be able to get to around 2000m-2500m that way, or deeper by using heavy ruin walls(which are pretty much indestructible).

If you going into the games library and modify the content of the LevelGenerationParameters.xml file you can adjuct the maps size, and content (main path, small tunels number and length, number of ruins on it ect.) for example right now for me the shortest trip is about 5klick or so. Of course its takink a bit longer to load in, but its fine by me.