I think it's almost certain that the Scandinavian rock paintings depict skin covered boats over a wooden frame. Dug-outs (or log-boats as they are now called) clearly came first but were very laborious to make requiring fire and lots of chopping out. The result is heavy and rides low in the water unless the hollowing out is extreme leaving very thin walls. Planks are not difficult to obtain using wedges to split logs, but the real skill is getting them to fit before sewing them together with root fibres. Then you have to caulk the gaps. A flexible frame is the simplest and most effective and when used with skins or leather, the result is very light, flexible, buoyant and water proof. Thor Heyerdahl certainly proved the seaworthiness of reed boats from Africa and America, but did we ever have them in western Europe?