They use Roplene construction which is a form of rotomolding. Here is what wiki says:
Roplene® has five times the impact resistance of fiberglass. Roplene® is a homogenous polymer, rather that a layered material like fiberglass, the color is solid throughout the material allowing easy repair of scratches and scrapes using sandpaper. Roplene® is a thermoplastic and can be welded like steel or aluminum. Roplene® is inert and will not chemically react to seawater, therefore will never blister, peel or fade. Roplene® is inherently flexible and absorbs vibration and shocks. Roplene® is inherently buoyant and floats unlike fiberglass or aluminum
