Real world stuff in service:
North Korea: Hi Speed Submergible Boat
http://www.strange-mecha.com/ship/M-Sub/N-Korea01.htm
http://www.fas.org/man/eprint/anti-sof.htm
Quote:
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North Korean naval forces have significant numbers of watercraft of various sizes and capabilities allocated exclusively to a maritime infiltration mission. These craft include a variety of submarines, coastal patrol craft, high-speed semi-submersible craft, air-cushioned amphibious craft, and rubber raiding craft.
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http://www.abovetopsecret.com/forum/thread281932/pg1
US stealth special forces boat, possibly submersible:
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http://www.aereon.com/pages/aereon26.html
March 1971 - Takeoff from the FAA Test Center, NJ