Corrected entry: When Stapanak gets to the engine room you can see that the rocker arms are not moving, But yet they are moving fast up to the tanker. (01:11:50 - 01:12:20)
Question: Would pouring whiskey into the tank have really thinned down the mix, giving the submarine more speed?
Answer: Adding alcohol to gasoline can lower the fuel's viscosity, thinning it to flow more easily. It can only marginally increase an engine's speed and power.
But the question was, would it have actually increased the sub's speed?
Thinning the fuel mix would not increase energy or performance, so it would not make the submarine faster.
Technically, yes, diesel with alcohol burns better. As for what happened in the movie: no. Whiskey is not alcohol; it's water with alcohol, and the water reduces the flammability a lot. Also, a sub has fuel tanks that hold tens of thousands of gallons; adding a few ounces of whiskey will not have any effect at all.
Corrected entry: When Admiral Graham and Admiral Winslow are in Charleston harbor waiting for Commander Dodge to arrive, Admiral Graham states that he's in line for a third star. Admiral Winslow then replies that this would no longer make him outrank Graham. Admiral Winslow's uniform has three stars on it, so if Admiral Graham did receive a third star, this would mean that he wouldn't outrank Admiral Winslow but that they would both have the same rank.
Correction: Exactly - that was Winslow's point. "Oh my god! I'll no longer outrank you." He never implies Graham will outrank him.
Even if Graham gets his third star, Winslow would have more "time in grade" as a 3-star and therefore still outrank him.
Correction: When underwater Diesel/Electric Subs secure the diesel engines and run on their batteries and electric motors. Since the diesel engines are shut down, one would not expect the rocker arms to be moving.
Fun fact: those are not rocker arms; you're seeing the crankshaft. For some reason, they mounted the engine upside down, probably because it's easier to mock a crankshaft turning than a rocker arm assembly with valves and all that.