Some SCAT hovercraft, especially the earlier models, have air plenum drain holes around the outside bottom of the hull. Typically, these holes are completely open which allows the entire air plenum to fill with water whenever the craft comes off of cushion. Not only does this reduce the craft's flotation, but it causes the craft to take a long time to get on cushion from a water start.

These one-way valves allow water to drain from the hull as needed, but prevent water from entering when the craft is off cushion. 6 valves needed on most SCAT 1's.  Later models can have fewer drain holes or none at all. Greatly increases flotation and water start performance at little cost.