You know, Voyage to the Botom of the Sea.

It appears that someone in DARPA did too, because they are looking at the flying sub, though it does not look as cool as the original:

DARPA’s strategic technology office wants an aircraft that can reach a destination, whether it is 1,850km (1,000nm) away by air or 185km by sea or 22km travelling underwater, in less than 8h.

I want whatever it is that they are smoking.