My understanding from others on this forum is that the problem with running "decks awash" is one of safety. In running decks awash you have a similar problem to maintaining periscope depth, but with a lot less margin to play with and a dead watch crew if you get it wrong. Worse, the sudden change in flow patterns from no water on deck to water on the deck would make for very unstable hydrodynamics. I guess it was possible for a good crew in calm weather but much too dangerous otherwise.
The only historical accounts I am aware of are Gunther Prien in Scapa Flow and Herbert Werner in the Bay of Biscay (in the book Iron Coffins). See
https://uboat.net/forums/read.php?3,...4391#msg-34391 It sounds like the technique was to run with slight negative bouyancy and the planes at an up angle; but hard to see how this could be controlled, especially at high speed.
Gamewise I have no idea if decks awash actually makes a difference in game and whether it was considered when the modders were setting detection parameters. It's a bit like sneaking some unrealistic extra knots out of the boat with realistic hydroplane angles...