It's really speed dependent, at the correct altitude and RPM propellers can be 85+% efficient in air. Also, while boats can get 'free' lift in water at speed your much better off with a wing aka hydrofoil because drag is one of those nasty velocity^3 things that quickly dominates.

So while a 2mph sub is more energy efficent than a 200mph aircraft the sub is rather impractical even pushing the sub to 20mph means the aircraft wins.