Performance requirements call for an aircraft that can cover 1,850km by air or 185km by sea, or 22km underwater in eight hours or less. And this is not some miniature pool hopper; DARPA wants it to be able to carry a crew of eight and a 2,000lb payload.
Speculation on design suggests an old-school snorkel to provide air supply for the power plant while your flying fish is in submerged mode, but don't rule out nuclear power or dilithium crystal for that matter.
Be advised, difficulties with developing such a platform will arise from the diametrically opposed requirements that exist for an airplane and a submarine, DARPA helpfully points out. Your concept should not only identify the technological limitations that need to be overcome to produce a swimming plane, you also need to provide proof that it's doable.
"In addition to the conceptual design studies, performers need to outline experiments or computational models that will be used to demonstrate that the major technological limitations can be overcome," DARPA warns, while admitting that "prior attempts to demonstrate a vehicle with the maneuverability of both a submersible and an aircraft" have, unfortunately, been unsuccessful.