Is there a mod that adds a positive or negative buoyancy?
 

Is there a mod that adds a natural positive or negative buoyancy when there is no forward momentum?

As I understand, a ww2 sub either surfaces or sinks when there is no forward momentum. (I know it's possible to continuously trim the ballast tanks to maintain a certain depth, but as I understand, that was not an easy or typical procedure)

There is for SH3. However, some of the underlying mechanics have changed in SH4, and subsquently SH5 as a result. In other words, the same tweaks that worked in SH3 to achieve boyancy changes, do not seem to work, or work as they used to, in SH4, and i'm assuming SH5 as well since its built upon SH4.

You are correct! Making changes to SH5 for positive/negative bouyancy only makes the sub not dive to specified depth resulting in loss of speed as the submarine is constantly trying to obtain the specified depth so dive planes never zero out.
If you go all stop the sub does not rise/sink...:cry: