Wouldn't regard myself as an expert by any means, but I do have some info and knowledge that is relevant, so here is that. But first, some pictures you might find useful:
Next some info on why subs are slower on the surface:
Three main types of drag act on a body going through a fluid. The most obvious one is drag caused by the actual shape, ie. Structural Drag, which is why subs, aeroplanes, cars etc are streamlined in an effort to reduce this. Next you have Form Drag, which is caused by low pressure from the fluid passing over the surface of the vessel, this creates low pressure over the surface of the sub as the molecules speed up and are strung out further apart, thus reducing pressure (this is what an aeroplane's aerofoil exploits to create lift, the technical name being the bernoulli principle, you can demonstrate this yourself by holding the back of a spoon in running tapwater, the curved back of the spoon will be pulled into the flow). Last you have skin drag, this is where the boundary layer (i.e the thin stratum of water close to the sub's surface) can erupt into tiny eddies and bubbles; these disturb the laminar flow, which slows things down. There have been experiments to try and inject polymers into the water which hug the skin of the sub and make it more slippery by improving the laminar flow, but there's not much data on the success or failure of this. Western analysts thought that was what the twin lines down the surface of the 'Akula' were when they first saw them, of course they aren't, they are simply safety line attachment rails for crew members on the deck.
Anyway, what all that technobabble above basically means is, that the flow over the sub is hugely disturbed when it is on the surface, and that makes it prone to massive amounts of drag in the disturbed flow of the water. Movement through this turbulent water probably also affects the efficiency of the screw too, as that would not be getting a clean flow either. Also interesting to note is that a submarine actually gets smaller (and therefore less prone to drag, though not by much) when it dives deep, in the case of a LA Class sub, it apparently shrinks by as much as 750 cubic feet when going deep as the water pressure compresses it.
Some other stuff on the LA Class sub:
The LA class sub has five main ballast tanks, when all those are flooded, they add 800 tons to the boat's weight of 6,100 tons, so of course it sinks to a certain depth based on how much water is in the tanks, but typically, the boat is set for negative buoyancy and held at a specific depth by the planes (i.e. dynamically), this is so that it will not accidentally rise if it slows to a speed where the planes become ineffective. Typical dive angles incidentally, are between 3 and 5 degrees. As far as I'm aware the screw's pitch is fixed (but someone may know better, as they are generally a closely guarded secret), one interesting fact that I do know though is that screws with odd numbers of blades are quieter than those with even numbers of blades, as they are less likely to set up harmonic sound waves, which is why you most often see seven blades on a sub screw.
Chock