Hide hulls stretched over a wooden and/or whalebone frame. These ships, while sufficiently sturdy, were lightweight, and could be pulled up onto the shore (or ice floe), and overturned to provide a shelter. These boats persisted into neolithic times, and there are engravings of them in stone, across northern Scandinavia and Karelia.
I always thought some larger wooden ships could be built for the purpose of being turned over on land as a shelter. Not that I could figure out how to do that.