introducing my first submarine, U2 "unforgettable fire" - older people should know that album ;-)
was and is still a pain to get leveled, can't get it better then this atm (not too bad, does drive straight, just backwards and stopped it tilts a little bit to the one or other side...)  check out the video first if you like: https://www.youtube.com/watch?v=pS2aoCII7mk
you can change how deep it dives by changing the angle of the 2 internal propellers if you want, i found this angle the best for the waters around maria planet .... oh and don't forget to dive up, you still drown inside a sub... they need to work on that bug :)
edit: i changed the sub a little bit, so it dives faster and stays upright while not moving a little bit better