Sorry, I just took care of maximum speed. Now I will take care of the angle of motion.
Can you drop the .capx here? ...
No. That would be too easy
As you can see on the picture, I added two new actions in line 5 and 6 that will take care of the angle of motion.
Now if you wish that angle of motion follows smoothly angle of submarine rotation it should be the same as rotation of submarine (like in this example, 125). If you want some inertia you should lower it and it will feel like drifting with car. If you make it higher it will act strangely. Try to play with it to get the result you like.
also I would suggest that you add dt to acceleration and deceleration, otherwise it will act differently with different framerate
just ignore line 2, I put animation speed by mistake