This is how i operate the boat in single player .

[THE_MASK]

Starting the mission .
Check the game time on the clock in the command or sonar room .
Check hatch is closed .
All stop engines .
Go to electric engines .
Open forward and aft vents .
Check main ballast tanks are full .
Close forward and aft vents .
Check sub is at least at 20 mts .
Blow negative tank . (sub should stabilise depth level) (dont need to touch the dive planes if you dont want to) .
Check dive angle is at zero degrees.