The US Department of Defense (DOD) released its Expeditionary Advance Base Operations Wargame Toolkit and options in November 2016. The unclassified document is intended to illustrate the art of the possible and does not imply DOD endorsement or funded requirements. It does however show the direction of thinking of the DOD and is jam-packed with ambitious and innovative concepts as well as some based closely on existing systems.
Submersible Support Vessel
One of the most interesting concepts in the pack (for the Covert Shores reader!) is a submersible transport with very high surface speed (50 kt!!!). The SSV would carry up to 20 passengers or five medical litters, implying that it is a dry-sub not a wet-sub. That also means that it is quite large. And the high surface speed but slow underwater speed implies a submersible boat configuration, completely different to the illustration used:
The document notes that this type vessel could provide logistic support, act as a battalion aid station for multiple CLTs or provide an emergency egress points for a CLT. Matched with the float Kmax like helos it could surface and resupply the helos and receive casualties and then resupply and further transfer casualties when float planes were available.
Remarkably the document cites the North Korean Improved-Submersible-Infiltration-Landing-Craft (I-SILC) (See old Covert Shores Blog), which is also used by Iran and Vietnam. Most SILC are only semi-submersible but some later examples are fully submersible down to about 50 meters but have extremely limited underwater performance.
Main Article: 
Taedong-B 'Kajami' ('Zulfikar') submersible boat
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The US designed solution which comes to mind is the Marion Hypersub which was recently used in a US Marine Corps’ Ship-To-Shore Maneuver Exploration and Experimentation (S2ME2) Advanced Naval Technology Exercise 2017 (ANTX17), held at Camp Pendleton in California in April 2017:
Another possible contender if the concept does reach the requirement phase is Juliet Marine Systems who built the Ghost reconfigurable high speed SWATH vessel which has also been proposed as the basis for an Unmanned Underwater Vessel (UUV) named Guardian:
The concept may be missing a trick however. Fully submerging increases complexity, weight and cost and also imposes greater training requirements on passengers (emergency escape, underwater breathing apparatus etc). Semi-submersibles, while not as stealthy, may provide a more achievable way of filling this capability gap. USSOCOM already uses the SEALION Combatant Craft Heavy (CCH) which is approximately large enough to meet the requirement and is capable of even greater speeds (60kt+):
Main article: 
CCH (Combat Craft Heavy) SEALION and Alligator Semi-Submersible Boats
Other interesting concepts presented in the document include the Ghostswimmer covert AUV:
An automated landing craft:
An Unmanned Surface Vessel:
A Rail Gun:
And the Box O’Rockets multi-weapon container launcher, shown here on the helideck of a Littoral Combat Vessel (LCS):
Related articles (Full index of popular Covert Shores articles)
CCH (Combat Craft Heavy) SEALION and Alligator Semi-Submersible Boats
Specialized Marine Craft (MSC) Sepcial Forces intercptor
World survey of Special Operations Craft
WP-18 Tactical Strike Craft
Special Forces Jet Skis