TDW's Depth Keeping Fix allows you to hold depth down to 170m and is included with New UIs with TDC I believe (can probably find the standalone version if you're not using TDWs UI mod).
Below 170m though, even with the mod you can't keep depth. Not sure if this is a submarine 'realism' thing or just the game being picky. Anything over about half a knot is pretty much suicide when escaping DDs so speeding up to keep depth below 170 often isn't an option.
