Found this - appeals to my sense of not sure what :-) My italics !
Air-cushion vehicles are given a distinctive yellow flashing light in paragraph (b). The flashing (120 regular flashes per minute) all-round light is displayed only when the vessel is operating on its cushion of air. Air-cushion vehicles may operate at high speeds, may tend to travel a little sideways in a crosswind, and may not be able to turn quickly, depending on the design of the vessel.
A U.S. Navy regulation (Title 32 of the Code of Federal Regulations, section 707.7) permits the use of a flashing yellow light on submarines, but the flash characteristic is not the same. Also, submarines tend to be much quieter, do not generally travel sideways, and on occasion vanish.