SEAL Delivery Vehicle (SDV) Mk.8 Mod 0
The 'Gator Class' SEAL Delivery Vehicle (SDV Mk.8 Mod 0, also written Mk.VIII Mod 0) has been the backbone of USSOCOM's SDV capabilities since the end of the Cold War. It offers a discrete method for inserting and extracting special forces in harsh maritime environments. It can transport 6-8 combat swimmers and their gear from a submerged submarine to within swimming distance of the insertion beach. The type is only now being replaced by a new system, the SDV Mk.11 (aka Mk.XI).

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The US Navy's SDVs (originally Swimmer Delivery Vehicle, becoming SEAL delivery vehicles' in the 1980s after the forming of USSOCOM) have set the standard since the 1970s. The SDV Mk 8 was larger, faster and more robust than those in service in other countries (unreported Italian types accepted).
US Navy SEAL Delivery Vehicle SDV Mk8 Mod-0
The main three SDVs used by the US Navy SEALs from 1970s-90s. In the background is a late-model SDV Mk.7 which carried four men. In the middle is an early example of the SDV Mk.9, and in the foreground is an early SDV Mk.8 which carried 6 men. Thanks to CDR Tom Hawkins.

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More critically, the US Navy had submarines large enough to host it. The original Mk.8 was designed to fit inside the twin hangars of the USS Grayback special forces transport submarine. By the time the Gator Class entered service Grayback had been retired but multiple new submarines had taken her place. None of these were dedicated special forces platforms like Grayback, but instead could be fitted with a Dry Deck Shelter (DDS). Selected regular fast attack submarines could carry a single DDS. And converted ballistic missile submarines could carry two.
US Navy SEAL Delivery Vehicle SDV Mk8 Mod-0

In contrast the Soviet Union had its equivalent SDV, the Triton-2, but that was never integrated into host submarines in the same way. Other countries, notably France and Italy, produced impressive SDVs but they tended to be much smaller and were therefore optimized for different missions.
US Navy SEAL Delivery Vehicle SDV Mk8 Mod-0
Britain's SBS (Special Boat Service) adopted the Mk.8. The right-hand image shows an original Mk.8 in near-arctic conditions.

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Although a natural evolution of the original SDV Mk.8, the Gator Class was virtually all-new. Improvements in propulsion made more room in the back which allowed two more passengers to be carried.

As well as the original Mk.8, the Gator Class also replaced the specialist SDV. Mk.9. That two-man craft was designed for reconnaissance and attack and could be armed with two modified torpedoes. These were termed SWA (Stand-off Weapon System). SWA gave it an impressive inshore ship attack capability. But the SDV Mk.9 had already been partially retired by the time that the Gator Class came along and it assumed the ship /infrastructure attack role as well.
US Navy SEAL Delivery Vehicle SDV Mk9
Divers of the Deck crew prepare an SWA armed SDV Mk.9 for launch. Thanks to CDR Tom Hawkins.

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In fact the original SDV Mk.8 had already been exercised in that mode in the 1980s, particularly in connection to the ongoing Persian Gulf War. For this it carried up to 8 Mk.5 Limpet Assembly Module (LAM) mines instead of the SWA. The Gator Class continues this capability.
US Navy SEAL Delivery Vehicle SDV Mk8 Mod-0
A Mk.8 with UUV payload carried externally

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