Folding kayaks have more seams and pockets where water can enter; they are quite difficult to sink. The fact is, any kayak can sink if it fills up with enough water. In the case of a folding kayak, the watertight seams and bulkheads help keep the boat afloat even when it is filled with water. So, while a folding kayak may not be unsinkable, it is certainly much more resistant to sinking than other kayaks.