What!!!!???? No one ever used decks awash in real life!!!!????? Gamey????!!!! WTF!!!!! (that seems to be a trendy expression around here lately, just getting with the program.....
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Decks awash is a VERY useful thing in real life for limiting the profile and visibility of the sub and was used extensively both by the Americans and Germans. Both groups of captains thought that lowering the profile by fifteen feet was meaningful and useful.
Yes, being heavier made no difference in speed, it would have slowed acceleration, but its most important effect was that the MBT already had lots of water in it. And negative was flooded, giving the sub instant negative buoyancy when they opened the top vents. In decks awash the lower vents were already open to let water in and only air pressure in the tank kept more from entering. What slowed the boats down, and that is replicated in SH4 is the increased wetted surface of the hull. With a Balao it's impossible to go more than 10 knots decks awash. That's good because above 10 knots the enemy spots you from a much greater range.
When you are fully surfaced, not only is your profile 15' higher, but you have to let thousands of gallons of water into the ballast tanks in order to submerge. You can cut your submergence time by more than 50% by running decks awash and believe me they did it a lot in the war.
It's a shame that we can't change from electric to diesel power. There were times when the real boats did that even when fully surfaced. We should be able to also. We also only have the choice of going to full charge batteries or normal running. We should be able to select which engines will drive and which will charge, as the real captains did.
When you're out on the ocean, there is phosphorescence everywhere due to wave action. Unless you were leaving a wake at high speed, in most circumstances you would not have to worry about phosphorescence being visible around the boat.
If it were calm, you would even have to worry about phosphorescence from disturbed surface water if you were at periscope depth and for some depth below that. It was normal procedure to submerge to 150 feet or so to do a high speed underwater run, partly for that very purpose. Of course, SH does not replicate that effect.