two questions I saw are:

1. what is "Decks Awash"? In game, you can run at flank speed with the decks completely under water and just the conning tower showing. However, it seems that was impossible in RL as discussed above. "Decks awash" seems to be having the MBT flooded so the deck level is lowered, but still above water.

2. As to when it was used, the advantage would be making the hull and tower harder to spot and lowering diving time. As a tradeoff, speed is lowered making it harder to escape on the surface. This is modeled in game, if you order a depth of 25 feet which basically brings the deck to just above water, flank speed is cut from 21 to 13 knots.

In RL, I could see two situations where "Decks awash" would be used. Either a night surface attack or as in U-569's case above, in daylight where there are a lot of aircraft around.

Decks awash certainly cuts speed down in my set up. If I use it, I don't go as deep to cut the engines off and my crew is on stations. I know many go deep enough that the electrics cut on, but technically the sub is dived, just you can man the bridge yourself.

Seems my speed at flank is about 11 kts.