I'm not sure exactly how this is supposed to work - I tried changing the surface depth setting to 7.something in the .cfg file too a while back, and didn't notice anything different for a while - every time I surfaced by just hitting "S" it would still take me up to the usual/previous fairly high 5.something or so.
Just last night though I decided to see if I could try running with decks awash (cruising near Dunkirk in daylight - figured I might need to dive in a hurry) and just clicked "7m" on the depth dial - sure enough, I was running with decks awash (looked cool from the external camera, too

) and my guys were still on deck. Also noticed that my speed did decrease a knot or two while running this way.
It was late and I didn't get a chance to test dive to see if it helped reduce dive time (I've read elsewhere that it does that), nor did I have time to try putting my guys on the deck gun just to see how funny that would look - may try that tonight