As far as I can gather, the evidence for the Scandanavian rock carvings portraying skin boats is fairly scant and based upon carvings that portray boats with possible 'pegs' on their frame. The 'pegs' have been interpretted as being used to stretch the skin across the timber frame. I guess an alternative interpretation could be that the 'pegs' are rollocks - the anchors for oars or even stiffeners/ribs for the hull (the double entendre possibilities are endless).
I have seen a number of Scandanavian boat carvings and what struck, was a significant number of carvings show boats with high stem and stern posts. These boats reminded me of the beautiful merchant ships that the Phoenecians were sailing around the Mediterranean during the Bronze Age.
I very much doubt that these carvings portray Phoenecian ships as they do not show masts or sails but it's worth looking at the technology that was available in the Mediterranean during the Bronze Age when trying to imagine what sort of vessels were plying our waters during the same period and earlier.