Subs are not energy efficient. Water is 1,000x more dense than air, & moving through it takes far more energy.
And, hybrid (land+water) is going to require far more hardware, driving up the price & weight (& thus efficiency) of a vehicle.
But, it's cool & he has the budget & gumption for it, so THANKS Elon, for keeping us entertained!
Yet.
I don't disagree that this is impractical, and you could be right that subs are inefficient, but the density argument alone doesn't seem sufficient, because propellers (and presumably other means of propulsion) also generate a lot more lift in water than in air.
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.
2 mph might be fine for a billionaire's private submarine yacht. If you're a billionaire and you're in a hurry, just transfer to the private jet.