"Underwater Express" - Supercavitating delivery sub?

Contract let here:

The United States is looking at the viability of a propulsion technology that could lead to underwater military transports capable of speeds up to 100 knots.

The U.S. Department of Defense's Advanced Research Projects Agency has awarded a $5.7 million contract to General Dynamics Electric Boat for a feasibility study into a vehicle dubbed the Underwater Express. The envisioned vehicle would be used to transport high-value cargoes or small groups of personnel from ships to shore in "littoral" coastal areas.

The Underwater Express is based on "supercavitation," which surrounds the vehicle with a giant air bubble that General Dynamics said allows an object to move through the water at speeds much higher than a standard submersible.

The current project is related to the movement of cargo and commandos. However, defense analysts note that supercavitation has enormous stakes for naval warfare. If successfully deployed, the technology could lead to supersonic torpedoes and high-speed submarines that could give destroyers and even sub-hunting aircraft a run for their money.

More info here:

For those unfamiliar with the term, "supercavitation" is usually used to refer to items like Russia's 230 mph Shkval torpedo, which uses hot plasma gas shot out of its nose to create a "bubble" with less resistance than the water around it. Similar principles apply to other approaches - place a bubble of water vapour or gas inside a liquid, and find a way to ride in it. Less resistance means less drag, which equals higher speed - IF you can remain in the bubble. This problem becomes especially acute when trying to maneuver, but may also apply to things like hitting certain underwater natives, et. al. Since water offers about 1,000 times the drag resistance of air, suddenly hitting that resistance at ultra-high speed tends to be rather messy.

While transitions from normal underwater travel into the supercavitating regime and back out again can be accomplished by gradually expanding and contracting the bubble, reaching supercavitation speed takes a lot of energy, and maneuverability that would both force and require the bubble to change shape is a far trickier challenge. The Shkval is not believed to have maneuver capability, for instance, leading to speculation that its original tactical concept may have involved firing a Shkval equipped with a tactical nuclear warhead into a US carrier battle group.
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While the USA is researching torpedo-like applications, therefore, the main focus of its supercavitation defense research is currently on the RAMCIS system, a 30mm gun with supercavitating ammunition that would be carried aloft by an MH-60S Knight Hawk helicopter and used to destroy shallow-water mines. This is difficult without supercavitation, as water tends to destroy most ammunition fired into it and will slow down even high-angle of fire bullets. RAMCIS is part of the overall AMCM (Airborne Mine Counter-Measures) program, and is expected to play an important role in version 1+ of the Littoral Combat Ship's MIW mission module. Other American supercavitation programs exist, but RAMCIS is definitely the most visible and the one with the highest open priority.

DARPA's Underwater Express program offers a very different focus - and a much harder challenge, since any manned supercavitating system would have to be maneuverable and the Underwater Express program explicitly calls for something that is "controllable at speeds up to 100 knots." At those speeds, the margin of error for a manned craft will be slim to none; "DARPA-hard" could end up describing a lot more than the research if something goes wrong with a production system. The total lack of acoustic stealth also makes one wonder about the manned supercavitation concept for special forces insertion et. al. in the oceans, while the dangers of navigating high-traffic large rivers at those speeds make one wonder about the concept beyond the oceans.

Remember that Iran demonstrated what appeared to be a Skyval torpedo during one of its recent war games. See here. DARPA announcement for bid proposals for Underwater Express here:

Through this program, DARPA is seeking to facilitate new operational opportunities in the underwater battlespace. Also relevant are the current limitations for small high-speed surface craft which suffer performance degradation in waves and are subject to exposure, while underwater alternatives today are very slow. The military advantage of very high speed underwater craft has yet to be exploited to its full potential because significant technological breakthroughs are needed for operational viability of such a craft. The ability to attain substantial underwater speeds for this class of vehicle is severely constrained by the power required to overcome the large drag forces on an underwater body. Without a radical means to solve this problem, strides that could be made in underwater propulsion are limited, as are breakthroughs needed in underwater sensing, navigation, and communications, all critical to taking strategic advantage of the underwater space.

Supercavitation offers 60-70% reduction in total drag on an underwater body. It can be attained by going fast enough to develop a full vaporous cavity, or it can be induced at lower speeds by injecting gas into a partially-developed cavity. Although the technology has been applied to weapons with minimal control capability, its application to larger vessels with transport missions will require thorough development. Our goal is to achieve tractable management and control of the dynamics of a supercavitating underwater body so that an eventual system, manned or unmanned, could be envisioned to travel in this state.

When you really, really need speed and aren't too worried about obstacles between you and the beach.... Maybe the "flying sub" from Voyage to the Bottom of the Sea could be a model.