-- Last Updated: Dec-19-12 4:08 PM EST --
Virtually all whitewater kayaks are made of rotomolded single layer polyethylene. Multi-layer poly is actually also rotomolded, but the polyethylene powder is loaded in stages, and the second load is formulated is such a way that it forms a foam core which adds stiffness.
The only problem with single layer polyethylene (solid polyethylene) is that poly is rather flexible. Until recently, few canoes were made of single layer poly for that reason but since kayaks have decks they are essentially tubular and that geometry adds inherent stiffness. Many poly kayaks also have internal foam pillars or bulkheads, which add additional rigidity.