Under the heading of 'but this does not make sense': modern submarines travel faster underwater than when traveling on the surface. No tricks or gimmicks, all else being equal, equal amount of thrust (Easy Boys! Easy Seamen!), same weight (displacement) but less surface area in the water so the logical conclusion is less drag, and that should result in greater speed, not less. But not only does it not work that way, it works very much the opposite way.
This was the major difference in submarine design for their first 50- odd years or so and modern designs; early submarines were really surface vessels that could go underwater for brief periods of time and move very inefficiently. Given any kind of choice, all subs through WWII and into the 1950's would always engage in warfare this way, contrary to common belief, and their underwater abilities were used simply to 1) hide and 2) run away when possible. In fact they were not even true submarines at all but rather submersibles.
It is all about the physics; a modern, cigar- shaped submarine hull is smooth, rounded and has flowing shapes, also called 'faired' in nautical or aircraft terms (that transitions to 'fairing' when used on motorcycles by the way). There are no sharp edges or projections to force the water into making a shock wave, rather the water just sort of moves away as the sub passes and then gently and slowly, closes back in to where it was before the sub got there, without actually accelerating any water in any direction. That does not happen on the surface because the part of the sub's hull that is above the water line is NOT smooth and does NOT make an even transition but rather forces the water out of its way; after passing, the water crashes back into the void left by the subs' passing.
There is a much easier way to prove this though without any mathematics, complex models, testing, modeling or anything else, and can be shown via a quick and simple set of examples: When a modern sub travels on the surface, it makes a bow wake, a stern wake and roils the water it travels through, exactly the same way ALL other surface vessels do. But completely submerged, there are no wakes and there is no roiled water left behind. So it takes the same energy to move the boat forward in either condition but when surfaced, it takes extra power to make those wakes and disturb (which really means accelerate) the water. So submerged, there is little to no water disturbance or acceleration to 'pay for'.
And the exact same thing can be done with craft that move in the air also, and it is not a new or even an 'off' idea, it was the original way to move objects a long way, efficiently, in the early days of air travel: a dirigible or airship. Look at the shape of a dirigible and a cigar- shaped submarine, they are virtually identical and for the very same reasons, a dirigible gently moves the air it is traveling through away and then allows it to again gentle return to where the airship had been without any sharp or sudden acceleration. Airplanes, on the other hand, accelerate a great deal of air downward at very high speed as they pass and that is the best case scenario, in reality, most airplanes, old and the most modern, roils up tremendous amounts of air in passing through the atmosphere and again, it robs power from the engine(s) to make that happen. So believe it or not, a Zeppelin from the 1930's will move the same mass the same distance and use FAR less fuel than any airplane or helicopter in existence, modern or not.
Brian