Question: Whose picture did Dodge salute in his cabin when he said "live to fight another day"?
Answer: Looks like Mush Morton, but that wasn't his ship. The real name of the submarine used in the movie was the Pampanito (SS-383). Her captain was Charles Jackson. It might be his picture on the wall. I can't find a picture of him.
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.
Answer: Dodge says "We live to fight another day, Captain." suggesting that the photo is of the previous captain of the Stingray.