While it is likely that the large muscles in the back/buttocks/legs will propel you much faster than the muscles in your chest/arms, they will also likely consume much more energy in the process.
It is therefore not obvious to me that using one's arms for propulsion is "inefficient" when compared to using one's legs.
During the course of my public education in the decadent USA, the two typical sports claiming to "exercise" the most muscles were/are said to be swimming and wrestling. Proponents of those sports are often claiming that their sport exercises "all" the muscles. The best athletes in the world at each of those sports have probably learned to rest muscles when they are not needed, so they have better endurance and more power when they do need to use those muscles.
Bipedal humans under the effects of gravity for their entire life/evolution have developed large efficient muscles in the leg group, to provide the most efficient locomotion. With dive fins on the feet, those large efficient muscle groups accomplish locomotion with similar efficiency, especially compared to the stock human hands.
Backing up to the claim that swimming exercises "every" muscle, that would mean using the arms for propulsion in water must exercise "all" the remaining muscles in the body. The arm flailing, wing flapping, hand motion of a diver in rototiller body position, running "too" negatively buoyant to more than compensate for the upward component of the fin thrust is not much different than the breast stroke-ish motion of a horizontal diver using arms to supplement the forward motion of finning. When you reach forward with arms/hands and stroke down/back to provide propulsion/thrust, you are flexing "all" the muscles in the upper body, to move a small amount of water with your tiny little hands; ab's, pecks, lat's, shoulders, neck, etc. And for what gain; a little bit of extra propulsion compared to the energy/o2 expended.
For the "wing flapper" there is usually a significant improvement in air consumption when the flapping is significantly eliminated. That requires learning proper body position, buoyancy control and fin propulsion; they are all interrelated.
Starting in the first session in the pool, I tell my students (even intro's) that; I will be telling them when they "flap," telling them not to "flap" and then demonstrating how to not do what I am telling them to not do. The same goes for bending knees to "kick" rather than "finning" with mostly straight knees, and for when their body position is a rototiller body position rather than a formula 1 race car body position (head slightly lower than fins, straight back).
If they are not properly buoyant I demonstrate letting some air out while moving forwards or adding some air while moving forwards. In briefings from the get go they know if I point at them and then demonstrate something I am expecting them to copy me; monkey see monkey do, I'm the lead monkey. They are also told over and over that
this is all so easy that any half wit, half fit 10 year old can do it well.
Most are diving while gently holding a wrist before the turn of the first OW dive; the slow learners get it by OW dive 2. Stubborn know-it-all's have made it to dive 5; a couple +20,000' "alpinists" took completely full inhalations/exhalations, every breath, for all 4 dives (Everest mentality?). I made them pay for a 5th training dive where I told them if their profile was as sawtooth as on those previous 4 dives I would not certify them. They breathed like divers on that dive and thanked me afterwards.
It would seem that not all scuba instructors can teach.
