Unfortunatly, I believe you will be a little disappointed with any progess you will currently be able to make, with underwater vehicles, untill P3D provides better support for these underwater vehicles. The majority of equivalent features that work with aircraft, are not yet working with underwater objects.
It is an area in P3D that has great potential, but to date, there is really little support within the core engine, for submercible objects.
Hopefully, in the future, it will be possible to create a vehicle, that can travel both Underwater, on the surface, and in the air - but things seem a long way from there at the moment.
Underwater "Land Classes" would be an exciting addion !! At the moment, unless you laboriously populate your underwater scenery with Frame-rate hogging scenery objects, the undewater world in P3D is very empty and barren.
But adequate if you want to simulate James Cameron exploring the Mariana Trench
I am not trying to be negative here, but having gone down this path earlier this year, I felt obliged to share my findings, and hopefully save you from getting too frustreated, trying to get functionality to work, that does not yet exist in the core engine.
You could however, always drive your Underwater objects from an external flight model, but even so, there are still other limitations (like animation, mission controls and trigger points etc) that will limit you.
That being said, if you do have a breakthrough, I would be most excited to see what you come up with.