First, they are inherently seaworthy by design. This form of construction closely resembles that of kayaks of Northern native peoples, and it is what made them such seaworthy craft. Like their ancestors, modern skin boats and folding kayaks flex with the action of the sea rather than fighting its forces as a hardshell does. The soft sides of a skin or folding kayak also play a role in seaworthiness. They dampen the impact of waves and wakes, so you are tossed around less.
Stability is another advantage.
Most foldables made since the early 1950s have air tubes running along their sides called air sponsons. These tubes, encased in the soft sides of folding kayaks, provide unbelievable stability both in initial and final phases. The soft sides themselves also play a part in stability. No matter how taut the skin, water pressure forms small indents in the hull between long pieces of the frame along the entire length of the boat. These concave pockets tend to grip or take a bite in the water to slow and control any sideways tipping process caused by beam waves or wake or by your moving around in your boat.