When your diving, you have to remember the air has to escape the ballast tanks through small holes and the positive bouyancy has to reverse to the negative to get any momentum downward. It takes time. In game, I normally crash dive then come back up. It seems to be quicker in time...but I havent put a clock on it either.
Im my experience as a real Helmsman/Planesman
...on both early and late 688 Subs, there was a huge "dive time" difference between "Sail Plane" boats to the dive planes on the bow.
If my memory serves me right, the bow plane boats submerged much faster because of the help of both sets of planes (stern planes/bow planes) working together. In the "Sail Plane" boats, the sail planes did not come into full effect until the decks were completely awash with 10' of the sail under water.
I know that there is a huge difference here (especially in boat types), but i do recall everytime we dove, it was down first then back up to PD...never from the surface directly to PD. It was hard enough to keep it 5-10 feet below the surface w/out broaching.
We should be glad that PD in this game is pretty consistant. Our Helmsman/Planesman are awesome since (unless your in a sea state 3-5 storm) the top of the boats never broach the surface of the water.
Imagine if they put in a "chief of the watch " feature and you had to pump or remove ballast from front, rear or center to keep the boat level. In a sense, it wouldnt be a bad idea. Then you can pump out ballast especially if your taking in water from damage.
These boats have normally (1) pump...the newer boats had several...each controlled by the C.O.W.